Answer
a) True
b) False
c) True
d) False
Work Step by Step
An "if and only if" statement p ↔ q is true if both propositions p and q are true, or they are both false. It is false otherwise (remember that p ↔ q is equivalent to (p → q) ∧ (q → p) ).
a) is true since both propositions are true
c) is true since both propositions are false
b) and d) and are false since one proposition is true but the other one is false. For example, it is true that if $2 + 3 = 4$, then $1+1 = 2$ (this hold since the premise $2 + 3 = 4$ is false: the implication will be true no matter what conclusion we choose), but the converse is $not$, so the bi-implication is false
