Final answer:
The correct verb form is 'comen.'
Step-by-step explanation:
The correct verb form that best completes the sentence is comen.
Frecuentemente ellos comen juntos en la cafetería del centro comercial.
'Comen' is the correct form of the verb ‘comer’ for the subject ‘ellos,’ which means 'they' in English. The other options, ‘comía,’ ‘comíamos,’ and ‘comías,’ do not match the subject in terms of number.
Learn more about Spanish Verbs