Correct answer:
B. limpien
This sentence is given in the subjunctive mood. We know this because it uses que as a relative pronoun. We use the subjunctive to express desires, doubts, the unknown, the abstract, and emotions. So limpien is the conjugation of the verb limpiar for the third person plural in the subjunctive mood. Finally:
Yo recomiendo que ellos limpien la casa pronto
