(C) Error: younger | Correction: youth
"The younger" is incorrect as it refers to a comparison, not a group. "Youth" correctly refers to young people as a group.
"The younger" is a comparative adjective that requires a reference point (e.g., "younger than adults"). To refer to young people as a demographic group, the correct noun is "youth." Examiners look for identification of the grammatically/contextually wrong word and its precise correction.