Thanks for the question! So we’re told here that Casey doesn’t buy a medium yellow shirt, and then asked what must be true.
Theoretically, we have nine total shirt combinations. S = small, M = Medium, L = Large R = Red, Y = Yellow, B = Blue
SR MR LR SY MY LY SB MB LB
But we’re told in the game that there’s no SR or LB, so there’s really 7 combinations.
XX MR LR SY MY LY SB MB XX
Now notice something here. Remember that we can’t buy S if we buy L, and vice versa. Let’s say we buy an S. Then we can’t buy any of the L shirts. And we need to buy two more shirts. But there’s only one other S shirt available. This means we have to buy at least one M shirt.
Similarly, since it works in reverse, if we buy an L, we can’t buy any of the S shirts. Again, we need to buy two more shirts. But again, there’s only one other L shirt available, so we need to buy at least one M shirt. So no matter what, we have to buy at least one M shirt!
What does this mean? Well, let’s say we don’t buy MY. We know from above that we have to buy at least one M. So if it’s not MY, it has to be MR or MB! And that’s what (B) says, which is why (B) is the correct answer.
Hope this helps! Feel free to ask any other questions that you might have.
