I am currently preparing for an English exam and as I was practising I encountered this exercise, which confused me:

I’ve often wondered why (1) that humans started to explore distant planets, while we still know so little about

the oceans here on Earth.

In (1) you have 3 options: IT IS / IS IT / WAS IT

It's a question inside a statement, it does not have the question mark, so should the statement structure follow 'why'?

Why it is that

or Why is it that...

If someone could help me a little I would appreciate it, thanksss.