As suggested by
@razerblade,
who-is-required-to-file is best answered by a reputable source dealing with filing Canadian taxes. The answer to this question,
who-is-required-to-file, depends on CRA rules.
That said, for any year in which the individual did in fact file a tax return, it is most likely OK to check "yes" required to file. By actually filing a return, the individual has in some sense determined he or she needed to file a return . . . that is, was required to file for some reason . . . even if CRA rules do not absolutely mandate the filing of a return.
My strong sense is that only for years in which the individual did not file a return is it important to determine definitively whether filing a return was required.
Citizenship applicants do not need to be experts in tax law. A response based on an honest, reasonable belief is OK. But before checking "no" not required to file and "no" did not file, it is important to make a real effort to determine if CRA rules allow the individual to not file for that year.
Ultimately, for any year in which the applicant did file a Canadian tax return, checking "yes" and "yes," required to and did file for that year, will NOT, COULD NOT, lead to an error in the administration of citizenship law.
Obviously, checking "no" not required to file and "no" did not file, for a year in which the applicant was required to file under CRA rules, could lead to an error in the administration of citizenship law, since that year would count as a year in compliance with tax filing obligations when in fact the applicant was not in compliance . . . and calculating years the applicant was in compliance with the tax filing obligations is required for a grant of citizenship. So this would be a material misrepresentation for which the consequences could be severe.