The correct answer is (b), a primary group. A primary group is a small group of people who have a close personal relationship with each other. They share a sense of belonging and mutual support. Nuclear families are typically composed of a mother, a father, and their children. They are often considered to be the basic unit of society.
A secondary group is a larger group of people who are connected by a common interest or goal. They do not have the same level of personal intimacy as a primary group. Examples of secondary groups include work groups, clubs, and professional organizations.
An economic unit of persons is a group of people who work together to produce goods or services. They may be related or not related. Examples of economic units of persons include businesses, farms, and households.
A powerful institution is an organization that has a lot of influence over society. Examples of powerful institutions include governments, religions, and corporations.
In conclusion, a nuclear family is a primary group because it is a small group of people who have a close personal relationship with each other. They share a sense of belonging and mutual support.