So many questions could be answered with "It's a sunny day!" Why are you holding that parasol? What's the opposite of a rainy day? What's the weather like on the sun today? But what do you mean by, "the detail is that it should be focused on the word 'sunny'"?

If I have understood you correctly you need to imagine what question would be answered with "It's a sunny day!". Is that correct? I think it's a strange task because there are lots of possible questions that might provoke that answer. As other people have said "How is the weather (where you are)?" or "What's the weather like (there)?"... but none of those questions show you have understood the word "sunny". Those questions are too general. There is also an exclamation mark at the end of the sentence. What does "!" signify in English? Some use exclamation marks all the time, but really it should indicate surprise or emphasis. So, perhaps you need to think of a situation where the conversation/situation is surprising? And which also demonstrates you understand the word "sunny". Something like: Question: "Why do you think I don't need an umbrella and coat today?" Reply: "It's a sunny day!" BUT perhaps the best advice is to ask the person who gave you this task? If they are teaching you they will be happy to explain what you need to do.

"What's the weather like in Brazilia today?" "Is it a nice day where you are?" There are many ways to ask this question...

What's it like outside?

What is the weather like? What is the weather like there? What is the weather like where you are? What is the weather like today?