Answer:
The question admits two correct answers: b) Es pequeña and d) Es débil y delgada.
Step-by-step explanation:
In Spanish, there must be an agreement among the subject, the verb and the predicate in number and gender.
The verb and predicate it is found in the answers; and the subject it is found in the question, in which by asking for her looks, it is clear that they are pointing out a female in third person singular. So in order to agree with the subject, the correct answer has to be in the feminine third person singular.
This is why option b) es pequeña; and d) es débil y delgada are correct, for in both the verb "to be" ("ser") is conjugated in the third person singular: "es", and in both the adjectives are also singular, and both are in the feminine form indicated by the use of the morpheme -a. Note that when it is a masculine adjective the morpheme -o it is used.
Even when in option a) and option c) the adjective, in both cases, it is in singular and in feminine form, the conjugation of the verb is incorrect in order to agree with the subject. In option a) eres alta, the verb is conjugated in the second person; and in option c) soy alta y guapa, the verb is conjugated in the first person, and therefore none of this two is correct.