You should have the guide open to each specific form when you are filling out the application forms.
This question gives you a list to select from. The one you should choose is, Other. You will then have more information you will have to fill out on here. There, you will write that you came to Canada as a visitor 2 years ago, and have been out of status for 1.5 years since that status expired, or however long it has been since your visitor status as a legal visitor expired.
I would also write a longer, attached paper here to explain your story. Why did you allow your status to expire. Why didn't you leave and renter to restart your status? Why didn't you apply for an extension? Explain why you didn't apply for a restoration of status within 90 days of it expiring. If you don't explain that here, I think it should be addressed somewhere in the application. Lay it all out on the table so they don't have questions about the matter.