The mnemonic SOH CAH TOA is intended to remind you of the relationship between right triangle sides and trig functions.
Sin = Opposite/Hypotenuse
Cos = Adjacent/Hypotenuse
Tan = Opposite/Adjacent
__
Here, you're asked for cos(A), so you want to identify the adjacent side and the hypotenuse. The side adjacent to angle A is marked 15, and the hypotenuse is marked 25. Then the desired ratio is ...