Analyzing a high school math textbook would show that a significant theorem was contributed to it by which Greek scholar?

The significant theorem that is commonly associated with high school math textbooks is the Pythagorean theorem, which was contributed by the Greek scholar **A. Pythagoras**. However, if you are looking for a mathematician known for a significant body of work in geometry presented in a textbook format, then **C. Euclid**, known for his work "Elements," would also be a prominent figure often discussed in high school math.

So, if the inquiry is about the theorem specifically, Pythagoras would be the answer. But if it's about a comprehensive geometric framework, Euclid would be the choice. Depending on the context, both A (Pythagoras) and C (Euclid) can be correct, but I would emphasize **C. Euclid** as the scholar who contributed to the structure of geometry as presented in textbooks.