30) D

So the buyer will definitely not buy the first car.

So probability of buying it = 0

If the second car is bought, probability of it being the best = 1

If the third car is bought, probability of it being the best = ½ (car 2 could have been better)

If the fourth car is bought, probability of it being the best = ⅓ (car 2 or 3 could have been better).

Adding up these probabilities = 1 + (½) + (⅓) = ¹¹⁄₆

However, we now need to multiply (¹¹⁄₆) by (¹⁄₄) since each option had a (¹⁄₄) chance of occurring. We get a probability of (¹¹⁄₄)