(c)
$P\left( \frac{B}{A \cup B^{C}} \right) = \frac{P(B \cap \left( A \cup B^{c} \right)}{P(A \cup B^{c})}$
$= \frac{P(A \cap B)}{P(A) + P\left( B^{c} \right) - P(A \cap B^{c})}$
$= \frac{P(A) - P(A \cap B^{c})}{P(A) + P\left( B^{c} \right) - P(A \cap B^{c})}$
$= \frac{0.7 - 0.5}{0.8} = \frac{1}{4}$