Question

In Ancient Greece, Euclid contributed to a better understanding of

Answer 1

I would go with Mathematics.
hope that this helps you. =)

Answer 2

Mathematics.

Euclid of Alexandria, who lived around 300 BC,  was a Greek mathematician known for his notorious contributions to geometry among other fields of mathematics, his contributions were so important that he is considered to be The father of Geometry. One of his most commonly known accomplishments was that of being the first person to write a complete review of Geometry on his well-known and recognized masterpiece, Elements.

His legacy continues to be of an incredible value nowadays.