Ana, ¿a quién le hablas español?

a. les hablamos español a ellos.
b. juan me habla español.
c. tú le hablas español a juan.
d. le hablo español a juan.

The right answer is the D) Le hablo español a Juan.

Explanation: Just to elaborate a little on the answer, it can be added that Ana, the subject, has been asked "Whom do you speak Spanish?," so the answer should be "I speak Spanish to..." Option C is incorrect, since she is not the one replying ("You speak Spanish to Juan," it says). Option B is also incorrect, since she is saying that Juan speaks Spanish to her. Finally, in option A the subject and the indirect object are plural ("We speak Spanish to them," it says). The right answer is, therefore, the D. "Le" is a personal pronoun in dative case that refers to the person that Ana speaks Spanish to.