Concept: The question tests on basics of Selection.
At least 3 men in the question implies 3men and 2 women Or 4 men and 1 woman Or 5 men and 0 women.
The Or condition implies an addition here after every case is computed.
(7C3)*(6C2) to select 3men and 2 women or
(7C4)*(6C1) to select 4 men and 1 woman or
(7C5)*(6C0) to select 5 men and 0 women.
Adding all we have (7C3)*(6C2) + (7C4)*(6C1) + (7C5)*(6C0)
= 525 +210+ 21
= 756 (Option D)