'Simple' is an adjective. It modifies 'it'. 'Simply' is an adverb. It would modify 'is' in a correct sentence. Word order usually isn't important when it comes to adverbs, but in this case you have to say "It simply is". This is the same as "It just is". You can also add an object. "It is simply not my problem". The important thing to remember is that the adverb modifies the verb, not the noun. As a general rule, you can remove an adverb from a sentence and it will still make sense.

simple is an adjective, so simple.

Simple is the correct option.you need an adjective.simply is an adverb.you need something else yo use simply. Eg. It's simply a dog.

*please