Complétez au présent. Elles (manger) beaucoup.

The subject is "Elles" (they, feminine plural), which corresponds to the third-person plural in French. The verb "manger" is a regular -er verb, but it has a stem change in the "nous" form and a spelling change in the "je, tu, il/elle/on, ils/elles" forms. For "Elles", the correct conjugation of "manger" in the present tense is "mangent". Therefore, the completed sentence is "Elles mangent beaucoup."