To know I would need to know what the verb in the parentheses is.
tomás está leyendo"Don Quijote."
Answer: Cuestan.
Explanation: Cuanto means How much in english, so the question is asking how much those books are going to cost. Whenever you see a Cuanto in a question like that, assume it's talking about a number of something, or money. So Cuestan means cost, in the terms of "How much does this item cost?".
las muchachas son muy lindas .
