The <em><u>correct answers</u></em> are:
A-theorem
; B-corollary
; C-Postulate
; and E-Definition.
Explanation:
In a proof, we are tasked with proving one statement using other information that has already been proven.
Theorems are statements that have already been proven, and are used to prove other statements.
A postulate is a statement that is assumed to be true, and is used to prove other statements.
A corollary is a direct consequence of a proven fact, and as such, are used to prove other statements.
A definition is an absolute truth, so it can also be used to prove another statement.