"I know because I saw it."
Translation:Я знаю, тому що я бачив це.
Should not both the first and third answers be correct, the only difference being the gender of the verb?
I can't see which answers you've got, but «Я знаю, тому що я бачила це» should be accepted too.
(Also, I'd move це before бачила because it sounds better if the pronoun object precedes the verb.)