a. on
b. in
ANSWERS:- a (on)
My birthday is on the 11th.
We write a note about it on top of--or over--the day-square (on the calendar marked) 5th)
2. b (in)My birthday is in August.
How to remember this: If we do not know or give the exact date, we know that the birthday must be inside (in) the time--the month--of August.