。
Can she play the guitar?
Literal
She [topic-は] guitar [subject-が] can-play [question-か]?
弾ける is the potential form of 弾く, formed by the standard u-verb potential pattern (~e + る): 弾く → 弾ける ('can play'). Notice the particle shift to が before the potential — 'guitar' becomes the grammatical subject of the can-be-played predicate, since potential constructions are stative in Japanese and stative predicates take が. This が-marking is the textbook signal that you're in potential territory: ギターを弾く (active 'plays guitar') vs ギターが弾ける (potential 'can play guitar'). The polite ~ます keeps the question in everyday-polite register.