A fact is said to be not ---- when it is neither proved nor disproved.

The question is asking what we call a fact that has not been proven or disproven. The options to choose from are "false", "existing", "proved", and "true". The correct answer is "false", which means that a fact that has not been proven or disproven can't be considered true or false until it is confirmed one way or the other. Therefore, in order to avoid confusion, it is called "not false".