No sé si compro este departamento. a. es mejor que compres este departamento b. es mejor que comprar este departamento. c. es mejor que compremos este departamento.

Right a. Es mejor que compres este departamento

The first sentence no sé si compro este departamento stands for the first person singular and this tense is given in the simple present. So compro is the conjugation of the verb comprar that matches the first person singular in this tense. So the other person must answer using the second person singular, hence compres is the conjugation of the same verb for the second person singular but in the subjunctive mood. Finally:

- PERSON 1: No sé si compro este departamento. a. Es mejor que compres este departamento.

- PERSON 2: Es mejor que compres este departamento

1-b si, conocemos a tu amiga maria2-d saben

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