This is a tricky question, since it depends what usage of the words is meant, but the best answer is: a fact.
He is treating the topic of whether the global warming is real or not and all our evidence strongly points to the fact that it is, in fact, real.
In the common usage of the word, we are talking about a fact - the best answer is b).
However, in science, the facts are referred to as "theories" - even gravity which we all experience, is known as a "theory", so there are also good arguments for the option c).
However, as I said, in the common understanding of the word, these theories are facts - if we disprove them, we stop calling them theories, and they are only hypotheses.