The mnemonic SOH CAH TOA reminds you that the cosine relation is ...
Cos = Adjacent/Hypotenuse
The triangle similarity statement tells you angle Q is congruent to angle N. The side adjacent to angle N is side MN, and the hypotenuse of triangle NPM is NP. Then the desired ratio is ...
cos Q = MN/NP . . . . . . matches upper left choice
