Which amino acid would, when incorporated into a polypeptide chain (not at the n or c terminus), make the charge of the polypeptide more positive?

Arginine.

Explanation: Arginine is an amino acid that is used for the biosynthesis of protein. It is made up of an α-amino group, an α-carboxylic acid group, & a side chain consisting of a 3-carbon aliphatic straight chain ending in a guanidino group. At physiological pH, the carboxylic acid is deprotonated (−COO−), the amino group is protonated (−NH3+), and the guanidino group is also protonated to give the guanidinium form (-C-(NH2)2+), making arginine a charged, aliphatic amino acid.