Respuesta :

ktreyb
cosine of an angle is the adjacent leg divided by the hypotenuse. 

So cos(60) = a / 12

I'm going to assume it's your last option because it's not in the picture. 
Yes SOH-CAH-TOA, Cos= Hypotenuse+Adjacent, so, cos(60) a/12 would be the correct answer so D.