Sandra lives with her two children. These three people represent

a. a nuclear family.

b. an extended family.

c. a cohabiting couple.

d. a nonfamily group.

A nuclear family.

Explanation:

A nuclear family is a family that consists in two parents and their children.

First of all we would have to know if Sandra has a partner or not, as the statement reads it appears that she doesn't have a partner.

In this case, Sandra and her children would actually be a single-parent family, but the nuclear family is the definition that it's closer to the actual definition as the other three refer to other kinds of families or people that are not actually considered family.

For example, an extended family is composed by members of your family but that are not from your immediate group, such as aunts, uncles, grandparents, cousins.

A cohabiting couple would refer to a couple that is living together but don't have children.

A non-family group refers to people that are not actual family, not even by law.

The most accurate term would be single-parent family if Sandra doesn't have a partner, a single parent is the person that lives with their children and who doesn't have a spouse or live in partner, there are many reasons to become a single parent such as divorce, break up, dead, adoption, etc.