Right answer:
Lee
This is a conversation between two people, in which A asks to B what is he or she reading.
Now, in Spanish, if you want to talk to a person in an educated and formal way, you should use Usted (abbreviated Ud.) instead of Tú (you).
Here, person A is making the question in a polite mode to person B using Usted (you).
In addition, in Spanish grammar, the verbs must be conjugated in accordance with the subject. In this case the verb leer (to read) in present tense is conjugated with the 2nd person in singular (formal) as:
(Usted) lee
Then, the conversation is:
A: ¿Qué lee Ud.?
B: Una novela de Isabel Allende.
A: What do you read?
B: A novel written by Isabel Allende.
The option (Tú) lees (you read) is incorrect because is informal; and the options cree (believes, 3rd person singular) and crees (believe) do not match with the context of the conversation