Which of the following errors does NOT belong to punctuation errors?

The correct answer, which indicates that incorrectly stating plural forms does not belong to punctuation errors, is grounded in the distinction between punctuation and grammar. Punctuation refers specifically to the use of symbols that clarify and separate written language, such as commas, periods, apostrophes, and colons. When discussing incorrect use of apostrophes, misplaced commas, or improper use of colons, each of these instances deals directly with punctuation rules—essentially the mechanics of written language.

In contrast, stating plural forms revolves around grammar rules that govern how words change to indicate quantity, rather than the symbols used to organize those words. Plurals involve alterations to the word itself (such as adding 's' or 'es'), not the application of punctuation. Thus, while it is crucial to be accurate with plural forms in writing, it is not categorized as a punctuation error, establishing why this choice is the correct answer in the context of the question.