Which of the following needs a proof?

Concept Used:-

Definition, Axiom, and Postulate are the self-evident which do not require any proof, while a Theorem is a proposition which required the proof for the establishment of its truth.

Explanation:-

From the given option we have to find out which needs a proof. Let's explain all the terms one by one.

1. Theorem- A mathematical statement is called the theorem whose truth need to be logically established also it has been proved
2. Axiom- A mathematical statement is called an axiom when it is assumed to be true and don't need to be proved.
3. Definition- It is a statement which is given for the meaning of any term.
4. Postulate- It assumes or suggest the fact, truth, or existence of something for the base of belief, discussion, or reasoning.

From the above definitions, we can conclude that a theorem requires a proof.

Hence, the correct option is 1.