So this question is basically asking "Of all the blondes, how many prefer morning classes?" For this, we have to add up all the blondes: 10 + 15 + 35 = 60.
Next, we see that there are 10 blondes that prefer morning classes. Using the pieces of info, we can form the probability as 10/60, which can be reduced to 1/6, or the last option as your answer.