"I know it."
Translation:Je le connais.
Je connais/sais ça should be the best answer. "Je le connais" should be "I know him" if it a person, but can be also translated to "I know it" if it is a book, or a movie for instance.
*Not a native French speaker
I don't think I'd ever say "Je connais/sais ça". It sounds off and very anglophone to me. "Je le connais/sais" sounds much better. If I used ça I would sooner say "Ça, je le sais" which would only serve to emphasize or point out something specific.
Maybe a third party can chime in.
I agree that "ça" really serves the purpose of emphasizing on something. It can be considered that "Je le sais, ça" or "ça, je le sais" is a bit better that "Je sais ça", although it can be said. Anyways, for next answers, I think it is pretty clear that "Je le connais / Je le sais" is the best possible answer. (Once again, without knowing the proper context).