。
How beautiful she is!
Literal
She [topic-は] what beautiful [explanatory-なんだ].
何て + adjective + なんだ is the casual exclamatory pattern — 'how [adj]!' / 'what a [adj]!' The masculine plain ~なんだ ending makes this feel slightly forceful, like an admiring outburst. Compare with the polite なんでしょう pattern (a more contemplative form): 何て works the same way as 何と but is more colloquial and conversational. In writing, exclamatory 何て tends to land with an exclamation point or emphatic punctuation.