Geometry

The answer is A. 16.

1. Analyze the congruence of line - segments in the diagram:

In a geometric diagram, line - segments with the same number of tick marks are congruent. Here, and both have two tick marks, so .

2. Use the property of congruent line - segments:

The property of congruent line - segments states that if two line - segments are congruent, then their lengths are equal. That is, if , then