Sociologists use the term marriage to refer to a. the set of norms that establishes and characterizes the relationship between married
individuals.
b. the married couple.
a social custom involving a wedding.
d. a group consisting of an individual, his or her spouse, and their children.

I think that it is A.

Explanation:

It couldn't be D, because not all married couples have children. It couldn't be "the married couple", just BECAUSE. And a social custom involving a wedding could not be it because not everyone will always get married. I think it is A because I think it fits best.

about the letter or who wrote the letter

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