Question

stem changing verbs e - ie, sa
ellos a jugar a las seis y media.

empiezan # empezamos # empiezo # empiezas

Explanation:

The subject "Ellos" means "they" (masculine or mixed - gender group). The verb "empezar" (to start) is a stem - changing verb (E - IE). The correct form for "ellos" in the present tense is "empiezan". "Empezamos" is for "nosotros/nosotras" (we), "empiezo" is for "yo" (I), and "empiezas" is for "tú" (you, informal singular).

Answer:

empiezan