Final answer:
The word that fits the definition of the belief in one god is monotheism.
Option B.
Step-by-step explanation:
The belief in one god is known as monotheism. This contrasts with polytheism, which is the belief in multiple deities, and pantheism, which is the belief that god is identical with the universe.
Atheism is the absence of belief in any gods. In reference to your question, the word that best fits the definition of the belief in one god is b. Monotheism.