。
She lives alone in an apartment.
Literal
She [topic-は] alone [as-で] apartment [in-に] is-living.
アパート is the lower end of the Japanese rental ladder — typically a one- or two-story walkup with thinner walls and lighter construction, distinct from マンション ('mansion'), the term for sturdier mid-to-high-rise concrete buildings (no English-style mansion implied). Both words are gairaigo, but their split meanings are entirely Japanese inventions. The same アパートに住んでいる + 一人で combination is the standard frame for describing a young person's first solo housing arrangement.