The question is what is the Irish for April, yet the month alone without the word 'month' is marked wrong. Are 'month of April' and 'April' interchangeable or not?
Note the difference between Aibreán and Aibreáin.
Aibreáin with a slender ending is the genitive of Aibreán (broad ending). You must use the genitive Aibreáin after Mí, and the nominative Aibreán on its own.
Aibreán and Mí Aibreáin should both be accepted as correct. Aibreáin is incorrect.