I shoulda/coulda/woulda done a lot of things during that down time, but did I?
That brings me to what "shoulda" is in actual words... is it "should of"? Sounds like it! But no, what you're actually saying is "should've," a contraction for "should have."
The correct answer is "b) I should have written my novel during shelter-at-home."
Note that the contracted form probably won't pass with most spell-checks or editors, and when writing more formally, you should spell out "have."
I should have written my novel.
I could have been a star.
I would have been the next big thing.
And never fear, it's not too late! You can still do it, whatever it is!