Pronouns vary the structure of sentences by: adding describers to nouns adding detail to verbs replacing nouns replacing verbs

It is B, adding detail to verbs.

A is describing adjectives

C is not right, prpnopuns do not replace nouns

D, pronouns and verbs are two different things why would one replace the other?

i believe it is the first choice (1.)

because the major theme is(“both the wild and civilization have rules.”)

and also as u said,( based on what buck learns within the call of the wild, "both the wild and civilization have rules".this is true of humans within society as well.