A box contains nine bulbs out of which 4 are defective. If four bulbs are chosen at random, find the probability that all the four bulbs are defective.

Answer: Option D

Out of nine, five are good and four are defective.
Required probability
$$\begin{array}{rl}& =\frac{{}^4{C}_4}{{}^9{C}_4}\\ & =\frac{1}{126}\end{array}$$