Answer: d. When he first enters the banquet hall, Macbeth appears cheerful.
Step-by-step explanation: Subject-verb agreement means that the verb and the subject must agree in number, this means that if the subject is singular the verb needs to be also singular, and the same applies when the subject is plural. We must be careful with the final "s" in the verbs conjugated in the third person of singular (he, she, it). In the given sentences the option "a" is incorrect because the subject is "Lady Macbeth" (she) so the verb should be "walks", in the option "b" the correct it should say "Banquo, not Fleance, is killed" because the subject is singular (Banquo), the option "c" should be, "Fleance, without Banquo, escapes" (because the subject is Fleance), and the correct answer is "d" because it says "he first enters" and "Macbeth appears".