Bebemos is the correct answer here because you have to conjugate the verb in the nosotros form. In Spanish, nosotros translates to we.
In the present tense, conjugated verbs ending in amos, emos, and imos are for the nosotros form.
Leemos is also in the nosotros form but if used, it will be in the wrong context.
Leemos means we read. "We read lemonade" does not make sense, but "we drink lemonade" is completely correct.
