Question

which line supports the theme that the customs of "civilized" society are confining and intolerable. The Adventures of Tom Sawyer

Answer 1

Answer:

"he had to talk so properly that speech was become insipid in his mouth"

Explanation:

This is the only detail/option that states how being "civilized" is not all it's cracked up to be.  The definition of "insipid" is "lacking flavor."  If Huck Finn is made to speak properly, he loses his boyish, informal manner of speaking.  This detail supports the theory that becoming a proper young man is stifling, confining, and intolerable.