Mark y Gloria [blank] los platos translates to "Mark and Gloria [blank] the plates". The verb to fill in the blank must be conjugated for the pronoun ellos, as the subject is third-person plural. The options are listed as follows.
- trabajan - "they work"
- limpiamos - "we clean"
- lavan - "they clean"
- doblais - not a word in Castilian Spanish (I think this is Spanish from Spain)
Mark and Gloria wouldn't "work" the plates, so option A is out. Option B is conjugated for the pronoun nosotros; we need a verb conjugated for ellos, so option B is out. As option D is not in Castilian Spanish, it is eliminated. That leaves option C - lavan; it is correct.
Mark y Gloria lavan los platos.