$(p \iff q)$
$= (p\to q)\wedge (q\to p)$
$= (\neg p\vee q)\wedge (q\to p)$ (Option B) As$(p\to q = \neg p\vee q )$
$=(\neg p\vee q)\wedge (\neg q\vee p)$ (Option A)
$=(\neg p\wedge \neg q)\vee (p\wedge q)$ (Option D)
So, answer C is not equivanet to $p \iff q$ and is the answer.