Which of the following postulates states that a quantity must be equal to itself? . a. symmetric. b. identity. c. reflexive. d. closure

Based on the mathematic definitions of these words, the answer would be B, Identity. In mathematics, identity is an equality relation. So A = B. They both have the same value as the other no matter what values (numbers) are substituted for the variables. 
Symmetric - one shape becomes exactly like another when you move it in a certain way (turn, flip, slide, etc).
Reflexive - something relating to itself. 
Closure - the act or process of closing, an ending, completeness.