**Answer: (C) \(\frac{2}{3}\)**

\( P\big(\frac{A'}{B'}\big) = \frac{P(A'\,\cap\,B')}{P(B')} = \frac{P(A\,\cap\,B)'}{P(B')}\)

= \(\frac{1-P(A\,\cup\,B)}{1-P(B)} = \frac {1-[P(A)+P(B)-P(A\,\cap\,B)]}{1-P(B)}\)

**\(= \frac{1-[0.5+0.4-0.3]}{1-0.4}=\frac{0.4}{0.6}=\frac{2}{3}\)**