Final answer:
The use of holophrases by a 6-year-old indicates unusual language development as children at this age typically use full sentences and understand more complex language rules.
Step-by-step explanation:
Among the options provided, D. A 6-year-old frequently communicates using holophrases represents a case of exceptional (unusual) language development. Typically, by the age of 6, children have moved well beyond the one-word stage (holophrastic stage) and are able to use full sentences and understand complex language rules. Holophrases are usually seen in much younger children, typically around the one-year-old mark, when they start combining words for meaning.
The case of exceptional (unusual) language development is represented by option B: A 4-year-old saying "mouses" instead of "mice". This is an example of overgeneralization, where the child applies a language rule ('add an s to the end of the word to indicate plurality') to an exception to the rule. In this case, the child is generalizing the rule to irregular plural forms. While the child understands the rule, they are still learning the exceptions.