。
How pretty she is!
Literal
She [topic-は] what pretty [explanatory-な] [I wonder/exclamation-だろう].
きれい ('pretty, clean, beautiful') is a na-adjective despite ending in い — one of the famous 'i-ending na-adjective' traps, alongside 嫌い ('disliked'), 有名 (which doesn't end in い but is na), 元気. The な before のだろう shows it's behaving as a na-adjective, not an i-adjective. なんてきれいなんだろう has the casual contraction なんだろう (from なのだろう), giving the exclamation a slightly more conversational, intimate tone than the polite なのでしょう version.