Can someone answer this question?

This is strictly my opinion, but I think the Phantom Menace is the Dark Lords of the Sith, namely Darth Sidious.

Well, there are two I thought could be possible--

a) Darth Sidious/Palpatine, since it was not yet revealed who he really was, but it was obvious he was behind it all.

b) Darth Maul, since phantom means "illusion" of "unreal", and most of the Jedi believed HE was the real threat. Of course, after his death they knew otherwise.

I would say Darth Sidious. He is controlling both the Trade Federation/Separatists and The Republic, while the Jedi do not know Palpatine is Sidious.

I am not quite sure but I'm guessing it could be Darth Maul of if u think about it they could be talking about Anikan. It is a good questin and those are my predictions but they're not quite accurate. Hope they help in some way :)

It is in fact, Darth Sidious, aka Palpatine.
He is a menace, obviously, to the Republic.
He is "phantom" in the definition of "not there of hiding." He is in fact both "not there" and hiding. He made them think Darth Maul was the real menace, while he was not. Thus he is both the "Phantom" and the "Menace."

The force