The correct answer is "player"
Step-by-step explanation:
In sentences including those that are interrogative the subject refers to the person, animal or entity that performs the main action or verb and that along with the main verb and the predicate or object creates a complete sentence and because of this it answers to the question "Who?". Also, the subject can be only one word or a compound subject with more than one subject or words on it. This implies in the case of the sentence "Which player earned the most points?" the subject is the "player" because this is the entity that performs the main action that is "earned" and the element that responds to the question "Who earned the points?". Therefore, the word that is the subject is "player".