What is a closet and what might be found there?

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.

quintessence, core, heart, crux, nucleus, substance, principle, fundamental quality, sum the origin of a word and the historical development of its meaning. insight, understanding, awareness, wisdom, education, learning, knowledge,