### If triangle $ABC$ is congruent to triangle $DEF,$ find the angle equal to angle $A .$

$\textbf D$

Step by Step Explanation:
1. Triangle $ABC$ is congruent to triangle $DEF.$
2. Remember: Two triangles are congruent if each angle of one triangle is equal to the corresponding angle of the other triangle, and each side of one triangle is equal to the corresponding side of the other.
3. Triangle $ABC$ is congruent to triangle $DEF.$

So, $A = D, B = E,$ and $C = F.$
4. Thus, the side equal to $A$ is $\textbf D$.