It might seem silly, but it's important to say that something is equal to itself. This applies to numbers like saying 2 = 2 and it also applies to geometric lengths as well. In line 5, we say that segment DB is the same length as segment DB. In short, DB is congruent to itself. This property is known as the reflexive property of congruence. You can think of a mirror giving off a reflection which has a similar word structure to reflexive.
You are correct if you were going to select choice B as the answer.
