He is a disinterested investigator.

A. uninterested

B. bored

C. unbiased

D. reluctant

Answer: Option C

Solution(By Examveda Team)

Uninterested : not interested in or concerned about something or someone.
Bored : feeling weary and impatient because one is unoccupied or lacks interest in one's current activity.
Unbiased : showing no prejudice for or against something; impartial.
Reluctant : unwilling and hesitant; disinclined.
Disinterested : not influenced by considerations of personal advantage.

Synonym of Disinterested is Unbiased