Answer:
"he had to talk so properly that speech was become insipid in his mouth"
Step-by-step 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.