"interpersonal relationships exist between people who are interdependent where one person has a significant impact on the other person"

Interpersonal relationships can vary in depth based on the nature of the relationship. Typically, the relationship is a strong, deep or close bond that two ore ore people have. This can be a short friendship or last for many years depending on the relationship. Due to the deep connect the people have with each other, they have a significant impact on one another. 

Ithink the answer you are looking for is (b).

Sector is best defined as (part).