¡la respuesta correcta!
a. piensen b. pensamos c. vuelvo

This is an order (taking into account the exclamation signs), a sentence written in present imperative mood.  

In Spanish grammar the Imperative is used by the speaker to give orders, requests, advice or make requests to one or more people directly. It is also used to exhort, incite or send another person to do something.

The forms of imperative in Spanish exist for all the personal pronouns except the 1st person in singular Yo (I).

In addition, note that for its conjugation the imperative is written without the personal pronoun and in present tense.

Therefore the complete sentence is writen with the verb pensar (to think) in imperative with the 2nd person in plural ustedes (you) as pienen:

Think of the right answer!

The other options are incorrect because:

-Pensamos (we think) although it is in present is not in imperative and is written with the 1st person in plural nosotros (we).

-Vuelvo (I come back) does not match with the context of the sentence