。
She is a very diligent student.
Literal
She [topic-は] very eagerly studies student is.
熱心に勉強する is a relative clause modifying 学生 — verb-final, no English-style 'who' connector. 熱心に is the adverbial form of the na-adjective 熱心 ('zealous, earnest'), formed by adding に. This shape is everywhere in formal description: 静かに歩く (walk quietly), 真剣に話す (talk seriously).