。
How beautiful she is!
Literal
She [topic-は] what beauty [explanatory-な] [I wonder/exclamation-のでしょう].
美人 ('beautiful person, beauty') is a noun applied almost exclusively to women in standard Japanese; the male equivalent for striking handsomeness is more often 美男子 (びだんし) or イケメン (a contemporary loanword-ish slang). The exclamatory frame なんと…のでしょう uses でしょう (the polite form of だろう) to add a refined, faintly feminine politeness to the exclamation — common in formal speech, narration, or stylised writing.