、。
Is she singing, or is she playing the piano?
Literal
She [topic-は] song [object-を] is-singing [question-か], or-else piano [object-を] is-playing [question-か]?
それとも is the disjunctive conjunction joining alternative whole sentences or alternative questions: 'A か, それとも B か' = 'A? Or B?' Different from や or か (which join words within a sentence), それとも is reserved for sentence-level alternatives. The verb 弾く ('strum, pluck') is the verb for playing string and keyboard instruments — pianos, guitars, violins — not the verb for wind or percussion. Both verbs end in the polite progressive ~ています, with か for question, giving a parallel structure that makes the choice transparent.