Writers often mix up these words, which leaves me scratching my head why.

Hypothesis is singular, as in The study’s results proved the first hypothesis to be true.

Hypotheses is plural, as in Researchers weighed competing hypotheses to determine which one merited testing.

A side note: In science, a hypothesis is a proposed explanation. It is tested and then the test(s) are duplicated by other scientists. Once several hypotheses supporting a theory are proven to be true, then the theory is thought to be true, at least in principle, as well.