Answer:
C
Step-by-step explanation:
It is provable about 180 ways, some of them quite elegant. There was a book published in the 1940s listing what they then knew. Others have been added since and it shows up in the oddest places which you can search out.
That is interesting but it does not answer your question.
A: Nothing is defined. This statement is a truth, but unproved. A is not the answer.
B: No term requires defining unless you don't know what a hypotenuse is, but that is a separate problem.
D: a postulate is a statement that is true that cannot be proven nor does it need to be. There aren't that many around in math. Euclid said "All right angles are congruent." True and not needing proof.
C: It is a theorem. This is a statement that may not be obvious and does require proof.
