Right answer:
D) bebas mucha agua
This sentence is given in the subjunctive mood. We use the subjunctive to express desires, doubts, the unknown, the abstract, and emotions. Subjunctive sentences often have parts linked by a relative pronoun (que, quien, como). So here we use the relative pronoun que. So bebas is the conjugation of the verb beber for the second person singular in the subjunctive mood. To contrast this, beber is an infinitive verb, bebes is the conjugation of beber for the second person singular in the simple present and bebo stands for the same but it stands for the first person singular. Finally:
Es importante que bebas mucha agua
