The girlfriend said, "It's hot, it's hot!" How should I reply to her?

It's hot now, of course it is. There is no need to reply at all. If you don't want women to sneeze, you will feel like a tornado. A small conversation will make you feel at a loss. It seems that you are making a mountain out of a molehill. Love is informal. For such a big thing, you don't have to think about how to reply in the conversation. Just make your girlfriend feel good with your own practical actions.