Look at the figure shown below which step should be used to prove that point a is equidistant from points c and b

D
is certainly wrong. You could extend the length of AD as far as you want and the two triangles (ABD and ACD) would still be congruent.

C
is wrong as well. The triangles might be similar, but they are more. They are congruent.

B
You don't have to prove that. It is given on the way the diagram is marked.

A
A is your answer. The two triangles are congruent by SAS