Answer:
1)
2) DEFINITION

3) THEOREM

4) POSTULATE

Explanation:
1) DIAGRAM B) A visual tool representing mathematical ideas to be interpreted
A diagram is the depiction or representation of information using symbols
2) DEFINITION A) A formal statement declaring the meaning of a word
A definition is a statement that outlines the meaning of a word or a group of words or a diagram, or a symbol or sign
3) THEOREM C) A mathematical statement proven using postulates and definitions
A general statement of a proposition that is by itself not evident, but given proof by a combination of postulates
4) POSTULATE D) A mathematical statement taken as a fact
An assumed truth taken as the foundation for further reasoning