Select one word for the given definition :
 A person who believes in the existence of God.
a)Atheist
b)Theist
c)Agnostic
d)Pacifist

Hint: One needs to be able to distinguish between the various terms related to the same concept.

Complete step-by-step answer:

 The given definition is that of an atheist. Option an “atheist” is a person who disbelieves or has no belief in the existence of God or gods, which is opposite in meaning as the given definition, hence is an antonym, hence it is an incorrect option. Option b, “theist”, means a person who believes the idea of the existence of a god or gods, specifically of a creator who intervenes with the happenings in the universe, hence it is the correct option. Option c, “agnostic” is a person who believes that nothing is understood or can be determined of the existence or nature of God, it is an incorrect option. Option d, “pacifist” is a person who believes that war and violence are unjustifiable, which is irrelevant to the given definition. Hence option d is an incorrect option.

Note: The tricky part of this question can be to differentiate between the words “agnostic” and “theist” because they both have nearly similar meanings and can be confusing.