It’s a hypothesis. We use "a" before words that start with a consonant sound. Since the first letter of the noun "hypothesis" (the h) is pronounced, we add the article "a".
It’s a host. The rule for using "a" or "an" is based on pronunciation, not spelling. Since the initial letter (h) of the word host is clearly pronounced, we use the article "a".
It’s a habit. The rule for using "a" or "an" is based on pronunciation, and not spelling. Since the word habit starts with a sounded "h", it takes the indefinite article "a".