Ed M. answered • 12/22/15
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I must disagree with Linda C. in that I do not believe that yesterdays is incorrect as the plural of yesterday (though it is certainly not standard if used as the possessive of singular yesterday, i.e., yesterday's, which sadly it is all too frequently--even by teachers, as in "yesterdays homework"). In fact, see Chris H.'s answer to your other question about adding suffixes to words ending in -y after a vowel to support why yesterdays as a plural is the correct spelling: https://www.wyzant.com/resources/answers/174077/english_summary/.
It is true that the use of yesterday in the plural is comparatively rare, but nonetheless it is possible and grammatical, as in "All Our Yesterdays," the title of an episode of the original Star Trek television series: http://www.startrek.com/database_article/all-our-yesterdays/. (How about that, I actually got a reference to the show in here; fellow Trekkers will be proud of me.)
Instead, of your other answer choices I would have to point to a), "agenda-agenda" as my choice for the correct answer to the question since the regular plural agendas does seem to be well attested.
