。
How beautiful she is!
Literal
She [topic-は] what beautiful [explanatory-の] [I wonder/exclamation-だろう].
なんと美しいのだろう uses an i-adjective (美しい, 'beautiful') as the predicate, with の to nominalise before だろう. This frame is more literary and formal than 美しいなあ or 美しい (with a simple sentence-final emphasis) — it's the kind of exclamation found in narration, poetry, or carefully composed prose rather than spontaneous speech. 美しい itself sits at a more elevated register than きれい ('pretty, clean') — 美しい風景 evokes sweeping or refined beauty, while きれい can describe an ordinary tidy room.