。
She took a deep breath.
Literal
She [topic-は] deep-breath [object-を] did.
深呼吸 (しんこきゅう) combines 深 ('deep') with 呼吸 ('breathing') for 'deep breath.' Like many compound nouns, it pairs with をする to form a verb phrase. A single deep breath often signals a moment of composure, preparation, or emotional reset in Japanese narrative.