Step 1: Read the sentence as a whole.
\textit{“She went straightly home.”}
Step 2: Identify the incorrect word.
The adverb “straightly” means \emph{strictly} or \emph{rigidly}, which does not fit the context of direction or movement.
Step 3: Use the correct form.
The correct word should be “straight”, which is commonly used as an adverb of direction (e.g., \emph{go straight home}).
Step 4: Final conclusion.
The error is in part \(\boxed{\text{(C)}}\).