。
She's a student.
Literal
She [topic-は] student is.
The plain copula form だ marks this sentence as casual — peer-to-peer speech rather than the polite ですform you'd use with a stranger. Same noun-predicate pattern as 看護婦です ('she's a nurse') but stripped to its barest, most informal form. You'd hear this from a friend describing someone, or in the kind of declarative sentence that opens fiction. The だ at the end signals 'I'm telling you this matter-of-factly' — bald and assertive, the kind of speech you wouldn't use with a superior.