The correct answer is: B. Soldier : Barrack
A principal is the head of a school. A soldier is a person who fights in a war. A barrack is a building where soldiers live. An artist is a person who creates art. A singer is a person who sings.
In each of the other pairs, the first word is a person who is associated with the second word in a specific way. A principal is associated with a school because they are the head of the school. A soldier is associated with a war because they fight in wars. An artist is associated with art because they create art. A singer is associated with a chorus because they sing in a chorus.
However, a soldier is not associated with a barrack in the same way that the other pairs are associated. A soldier may live in a barrack, but they are not defined by their association with a barrack. A principal, a soldier, an artist, and a singer are all defined by their professions. A barrack is simply a place where soldiers live.
Therefore, the pair B. Soldier : Barrack is the pair in which the words are differently related from the rest.