First,
@canvan is WRONG about needing or submitting Notices of Assessment.
Again, this is just plain WRONG.
Secondly,
mostly FOLLOW the INSTRUCTIONS. FOLLOW the CHECKLIST. Read and follow them as you understand them. (And use the most current versions available. The forms tend to change from time to time.)
There is NO requirement to submit any tax related documents. (Albeit IRCC may request some documents
LATER in the process if there is a reason why IRCC has concerns about the applicant's eligibility.)
Item 12 in the current citizenship application form CIT 0002 (06-2018) is straight-forward.
-- The applicant is required to state the applicant's SIN (with easy to understand exceptions).
-- The applicant is required to list the preceding FIVE TAX YEARS and complete the chart by indicating, for each of those tax years:
-- -- whether the applicant was required (pursuant to CRA rules) to file a return that year, and
-- -- whether the applicant did file a return that year
The requirement for citizenship eligibility is simply that the applicant must have complied with CRA tax filing obligations for at least three of the preceding five years. Thus, if the applicant has filed a return for three of the preceding five TAX years, the applicant meets this requirement. However, even if the applicant did not file a return for a given TAX year, if the applicant was not obligated to file a return (pursuant to CRA rules), the applicant checks "NO" and "NO" for such a year and that year also counts toward the THREE years of compliance.
TAX YEAR 2018:
Every year, early in the year questions are asked about the most recent tax year. Obviously, as of today (January 21, 2019) last year, 2018, is one of the five preceding TAX years and thus needs to be listed in Item 12. Many, probably most, will be required to file a return for 2018. BUT AS OF TODAY A "NO" NOT REQUIRED TO FILE response is appropriate for the TAX year 2018. There is NO requirement to file a tax return for 2018 until at least April 30, 2019 (for some there is no requirement to file until a later date; for example, those who are self-employed are not required to file a return for 2018 until June 15, 2019).
And, indeed, most prospective applicants who had earned income in 2018 will not be prepared to file a return until they have received information such as T4s, which many will not have yet.
So the tax year 2018 should be listed and as of today it would be OK to check "NO," not required to file, and "NO," taxes not filed, and this should count as one of the three tax years for which the applicant must show compliance with the tax filing obligation. (But see suggestions below.)
That said -- some suggestive observations:
Meeting the requirements meets the requirements. Sure, this is largely a meaningless tautology, but it is worth highlighting. We immigrants come to Canada with incredibly diverse scenarios and our lives in Canada encompass a wide, wide range of experiences and situations. So how to best answer many of the questions in the application form can be very PERSONAL, and WHEN to APPLY is likewise a very personal decision. For the prudent, cautious PR, just barely meeting the minimum requirements is NO WHERE NEAR GOOD ENOUGH. BUT more than a few among us have situations which do not allow for much flexibility.
So the WHEN to APPLY question is especially personal, a personal decision which should be made with due consideration given to many facets of the requirements and process. For example, a significant buffer over the minimum days present is prudent, at least some weeks extra in my view (I personally waited nearly two additional years to apply, but I had particularly personal circumstances motivating the decision to wait that long).
Relative to the requirement that the applicant have complied with tax filing obligations for at least THREE of the preceding FIVE TAX YEARS, obviously it is BEST if the applicant can truthfully report compliance for ALL five of the listed TAX years (which might include some "NO" and "NO" years (not required, no return filed years), especially for years prior to actually settling to live in Canada). Compliance for more than three of the relevant five tax years is NOT required, and it might not have much influence, but obviously full compliance presents a better picture, and many of us prefer to proceed looking our best.
Additionally, the further into the year it gets, the more one might want to lean toward filing a return for 2018 and being able to check "yes" and "yes" (yes, required, and yes filed) for 2018 . . . and, in respect to this, for many it might help to simply wait a little longer, if necessary, to do this (file the 2018 return before applying), thereby also adding some more buffer days for the presence calculation.