Either works, given the often-contrary nature of the English usage of H. However, I've heard "a hypothesis" more than "an hypothesis". Improved Answered: In instances where words that begin with "h"...
A hypothesis is correct grammar here's why: A hypothesis is correct because 'h' is not a vowel. for example in the fragment: a cat, c is not a vowel so 'a' applies. If on the other hand it says 'an'...