PLEASE HELP!! GEOMETRY

Explanation:

A conjecture is a guess. You cannot use guesses to justify statements in geometry, or in any logic setting. Sometimes you might hear in a court case that someone made a claim "based on speculation and conjecture" which are shaky claims at best. In other words, they made a claim that hasn't been verified.

A postulate can be used because it's a statement that doesn't need proof. It's something fairly trivial such as "two points form a line". So we can rule out choice A.

A previously proven theorem can be used as justification as well, since it was proven to be true. An example would be the pythagorean theorem. So we can rule out choice C.

Lastly, choice D can be ruled out because definitions can be used as justifications. An example of a definition is "all right angles are 90 degrees".