The explanation is incorrect.

First of all the LENGTH of the string representation of the months is EVEN, not ODD.

Second of all, the proposed method is NOT THE ONLY ONE another way.

So the answer should not start from BECAUSE, it should start from BECAUSE FOR EXAMPLE....

Another (not the last) way to extract is to JOIN the table stud_mast ON month(dob) with another table of subselect, or UNION, which contains the given numbers : 2,6,7,8,11,12.