Final answer:
In a two-column proof, the reasons used to justify each step or statement are typically theorems, postulates, or definitions.
Step-by-step explanation:
In a two-column proof, a reason is used to justify each step or statement in the proof. The reason is typically a mathematically accepted fact or rule, such as a theorem, postulate, or definition. Observations, on the other hand, may inform the steps or statements in the proof, but they are not typically used as reasons in a formal proof. Therefore, the correct options for reasons in a two-column proof are:
a) Theorem
b) Postulate
c) Definition