Probability that the cab is a green cab = 0.85
Probability that the cab is a blue cab = 0.15
The witness can correctly identify the cab colour only 80% of the time
So, probability when the witness is correct means when the witness identifies blue cab = 0.8
& Probability when witness is wrong = 0.2
We know, $P(E) = \dfrac{Numbe of favorable outcomes}{Number of all possible outcomes}$
The probability that the accident was caused by a blue cab = $\dfrac{(0.15)*(0.8)}{((0.15)*(0.8)) +((0.85)*(0.2)) }$
= $\dfrac{1.2}{2.9}$
= 0.41
= 41% option C)