。
She's busy with work.
Literal
She [topic-は] work [with-で] is-busy.
The cause/activity-marking で returns. 仕事で忙しい ('busy with work') is one of the most common Japanese phrases of daily life, slotting into countless conversational excuses, schedule explanations, and expressions of unavailability. 忙しい takes the cause/activity of busyness with で, not が or を — a particle pattern worth learning as a frame.