Identify which triangles are congruent, and explain your answer.

This is a 30-60-90 triangle. Following the pattern of x, , and 2x. So lets us look for that pattern in these triagles.
Judging from what I know, the answer is A and C. 
They hypotenuse is 2 times the shorter leg, which is evident here. B and D are incorrect, since if the longer leg is 18, then the smaller leg would be a large square root , possibly a number in the mid 500s being squared.
Hope this helps