by September or in September? [closed]

I'm preparing for my English grammar exam, and I faced this question

The weather is going to be hot ........ September

a. on
b. at
c. in
d. by

I'm not sure whether the answer is c or d

Solution 1:

A on September - wrong and meaningless. Although the usage "on a day in September" is accepted. "We went to the seaside on a day in September".

B at September - wrong and meaningless.

C in September = during the month of September

D by September = before and up to the time that the month of September starts

Which means that either C and D may be correct, depending on what is meant. This is a poor question if only one correct answer is expected.