Nick is admitted to a mental institution because he hears voices talking to him that no one else can hear, and he sees demons attacking him, though no one else could see anything near him.
nick's symptoms are known as select one:

a. delusions
b. obsessions
c. hallucinations
d. compulsions

c. hallucinations    

Explanation:

Hallucinations: In abnormal psychology, the term hallucination refers to the process of experiencing sensations by a person that may appear as to be real or true but it has been created in the mind of the person.

Example: Hearing voices, seeing things that aren't physically present.

Hallucination includes hearing, feeling, seeing, tasting, or smelling something that is not physically present.

In the question above, Nick's symptoms are known as hallucinations.

the answer would be in this answer. someone already asked the question.