In the triangle below, which is equivalent to sina?

1. By definition, you have:

 Sin(α)=Opposite/Hypotenuse

 2. Then, SinA is:

 SinA=BC/AC

 A is the angle in the vertice "A".
 BC is the opposite leg to the the angle A.
 AC is the hypotenuse of the triangle.

 3. By definition, you have:

 Cosα=Adjacent/Hypotenuse

 4. Then, CosC is:

 CosC=BC/AC

 C is the angle at the vertice "C".
 BC is the adjacent leg to the angle C.
 AC is the hypotenuse.

 5. Therefore, the answer is the last option:

 The equivalent to SinA is CosC

We have that

sin A=BC/AC

and 
cos C=BC/AC

therefore
sin A= cos C

the answer is cos C