。
She studies eagerly.
Literal
She [topic-は] [eagerly-熱心に] studies.
熱心に is the adverbial form of the na-adjective 熱心 ('eager, enthusiastic'). 熱心 literally combines 熱 ('heat, fervor') and 心 ('heart, mind') — 'heart-fervor.' Common in evaluations of work or study habits.