Little...

JadelCould you please tell me which one of these answers is correct?

Which one do you think is correct?

Little knowing.

JadelLittle knowing.

No. Before an independent clause beginning with "but" you'll need another independent clause. Only the last two choices fulfill that requirement because "knowing" and "known" are non-finite forms that won't work in an independent clause. You should also know that after initial "little" you'll need subject-verb inversion.

Would

CalifJimafter initial "little" you'll need subject-verb inversion.

"Subject-verb inversion" means that the word order is like the word order when you ask a question.
So you have only two choices...

Perhaps, the last one.
But I don't know what the sentence means?

JadelPerhaps, the last one.

Yes.

"Little did he know " is an idiomatic expression, usually followed by a that-clause.
It does not mean that the person knows a little bit about something. It means that they don't (or didn't) know anything about it. Or that he didn't suspect anything about something. He was clueless.

Little did he

Little did he know but I realized what he had done.
What does it mean?

JadelLittle did he know but I realized what he had done.

I realized what he had done, but he didn't know at all that I realized it.
He had no idea that I realized (knew) what he had done.

CJ