Our knowledge of the past is still largely a matter of informed conjecture.

Answer: Option C

Conjecture : an opinion or conclusion formed on the basis of incomplete information.
Guess : estimate or conclude (something) without sufficient information to be sure of being correct.
Position : a place where someone or something is located or has been put.
Certainty : firm conviction that something is the case.
Form : the visible shape or configuration of something.

Antonym of Conjecture is Certainty