Each multiple choice is worth 4 points, and X is the number of questions he get's right. For example, he gets 3 right: 4 x 3 = 12 points in total.
The same goes for the word problems. Y represent the the number of correct answers so, for example: he gets 4 right and each one is worth 8 points: 8x4
Since he needs to have a least 50 points, that means that he can have 50 or more than that. That can be written as: ≥
So, as i explained, the only possible option is D.
4x + 8y ≥ 50