Which biconditional statement is true? a. a number is divisible by 9 if and only if it ends in 3. b. a quadrilateral is a square if and only if it has four right angles. c. the sum of two integers is positive if and only if the two integers are positive. d. a number is an even number if and only if it is divisible by 2

Let us evaluate the given statements.
A, A number is divisible by 9 if and only is it ends in 3.
     23/9 is not divisible.
     FALSE

B.  A quadrilateral is a square if and only if it has four right angles.
     A rectangle is a quadrilateral with four right angles, but it is not a square.
     FALSE

C. The sum of two integers is positive if and only if the two integers are
     positive.
     12 + (-10) = 2 (positive).
     FALSE

D. A number is an even number if and only if it is divisible by 2.
     Thi is true by definition.
     TRUE 

D