SAT Question 1353b86e: Probability And Conditional Probability answer and explanation with Desmos solution

Choice D is correct. If a marble is chosen at random from the bag, the probability of choosing a marble of a certain color is the number of marbles of that color divided by the total number of marbles in the bag. Since there are $10$ blue marbles in the bag, and there are $40$ total marbles in the bag, the probability that the marble chosen will be blue is $\frac{10}{40}$.