Question

Tomorrow’s weather is forecasted to be mostly sunny with a gentle breeze.

Which type of sentence is this?

Answer 1

This sentence is a simple sentence, because it contains only one verb (is forecasted) in its finite form (there is also the verb to be, but it is infinitive, so it doesn't count), which means there is only one independent clause, which means this is a simple sentence.

Answer 2

Answer: (edmentum/plato)

This is a declarative sentence. Lets look at our choices,

imperative -

giving an authoritative command; peremptory.

declarative -

of the nature of or making a declaration.  

exclamatory -

relating to a sudden cry or remark, especially one expressing surprise, anger, or pain.

So it can only be a declarative sentence because it is not giving a command, nor is it relating to any emotion. The other person is wrong.