。
She quit smoking.
Literal
She [topic-は] smoking [object-を] stopped.
やめる is the everyday verb for quitting, stopping, or giving up — applicable to habits (タバコをやめる), jobs (会社をやめる), school (学校をやめる), and just about anything one ceases. It's distinct from 止まる (intransitive 'come to a stop') and 止める (transitive 'bring to a stop'); やめる carries a sense of voluntary, conscious cessation, often with a permanence the others don't. The verb is usually written in hiragana, though 止める/辞める also occur depending on the nuance — 辞める is preferred for jobs, 止める for stopping motion.