(c)
We know, $P\left( \frac{A}{B} \right) = \frac{P(A \cap B)}{P(B)}$
and $P\left( \frac{B}{A} \right) = \frac{P(B \cap A)}{P(A)}$
$\therefore P(B) = \frac{P\left( \frac{B}{A} \right).P(A)}{P\left( \frac{A}{B} \right)}$
$= \frac{\left( \frac{2}{3} \right)\left( \frac{1}{4} \right)}{\left( \frac{1}{2} \right)} = \frac{1}{3}$