。
She quit smoking.
Literal
She [topic-は] cigarettes [object-を] smoke [the act of-のを] stopped.
The pattern [verb plain]+のをやめる ('stop doing X') is the standard frame for quitting an activity. の nominalizes the embedded verb たばこを吸う ('to smoke') so it can be marked with を as the object of やめる ('stop, quit'). The result has the structure of '[the act of smoking]-を stopped.' Compare with the simpler たばこを止めた, which would also work but doesn't foreground the cessation of the activity itself. Note たばこ is conventionally written in hiragana or katakana — the kanji 煙草 exists but is uncommon outside set phrases like 火付け煙草 or older texts.