"What beautiful weather." is the ONLY correct answer. ('weather' is an uncountable noun, so you can also say " The weather is beautiful."

"What beautiful weather!" is correct. "What a beautiful weather" is incorrect. It has to do with which words are "countable" or "uncountable" nouns. Here are some other examples. In every case, √ means "correct" and X means "incorrect." √ Oh, what a beautiful morning! Oh, what a beautiful day! X Oh, what beautiful morning! Oh, what beautiful day! √ Oh, what fun it is to ride in a one-horse open sleigh. X Oh, what a fun it is to ride in a one-horse open sleigh. √ That is a spectacular sunset! X That is spectacular sunset! √ That is very clear water. X That is a very clear water. √ That is a very clear lake. X That is very clear lake.

"What a beautiful weather" is correct because "beautiful" is an adjective, so it needs a noun after it. However, it would be more natural to say "What beautiful weather". Hope this helps!