It would be A. You can find this out by using SOH CAH TOA,
SOH means sin= opposite/hypotenuse
CAH mean cos= adjacent/hypotenuse
TOA means tangent= opposite/adjacent
You would use CAH since you are trying to find cos∠C
You would have cos∠C=10/26.
26 is the hypotenuse since it is opposite the right angle and is the biggest side.
10 is the adjacent since it is right next to C
You would simplify this down to 5/13 by dividing by 2.
This gives you A.
