The answer is the 15th.
If you do a deep, head scratching analysis of Ides of March it's Ides + of + March = Half of March, which will give you fifteen days on either side of March, because March has 31 days.
This quiz was 30 minutes long. I spent 25 on this question, because I remembered 15 and got 16 by hand.
So that's why I got an A- in Sophomore English, admissions council. We studied Julius Caesar and I used my brain.