Segment ab is congruent to segment ab. this statement shows the property.
a. reflexive
b. symmetric
c. transitive
d. substitution

The answer is the option a. reflexive. Reflexive property means that the property applies over the same object. In this case means that a segment is congruent to it self. Other example of reflexive property is similarity. This is any figure is similar to itself. Also congruency of angles: any angle is congruent to itself.

C. expression because any expression or equation may contain more than one variable

Understanding what? lol