Hopefully this was on a calculator paper!
To work it out, you have to find out the total probability of picking the two marbles, so you find out how many marbles you have, 15, and times that by itself as you are replacing the first marble. Then do the number of black marbles, 4, times the number of blue marbles, 6, to give you the numerator.
Your probability is now 24/225 which is not one of your answers so you have to divide it by one of the denominators. I found that the top answer was the only one in which the denominator and numerator could both be divided to get an integer, whole number,making it the right answer.
