_ _ _ _ _ _ Today's weather! How beautiful! How sunny it is! What a nice day!

First of all, you can't use how here. If you want to use how, it can only be how good the weather is. How to modify Nice here.

How to express the degree and frequency of doubt or exclamation?

How old are you? How often do you go swimming? How cute you are!

What is used to modify the noun structure of nice weather? The emphasis is on "weather", not how to modify adjectives to express the degree of nice.

Secondly, weather is an uncountable noun, so you can't choose B.