Short answer C
Comment
How to explain this is not simple. There are all kinds of ways of dealing with it.
One fact is that Sin(A) = Cos (90 - A). But you may not have had that yet.
An easier way is just to use side notation.
The sin of an acute angle in a right triangle is the Opposite over the hypotenuse.
In this case, Sin(A) = o/h = BC / AB which you were told was 24/25
Now look at the Cosine of B
It's definition is Adjacent / hypotenuse
Cos(B) = a/h = 24/25
Answer
Sin(A) = Cos(B) or
C <<<< answer