Final answer:
Differences between words an infant understands and ones she doesn't can be identified in their vocabulary size by 20 months of age, reflecting significant cognitive development in language acquisition.
Step-by-step explanation:
By 20 months of age, differences between words an infant understands and ones she doesn't can typically be identified in A. Vocabulary size. At this stage in infant development, toddlers can use as many as 50 words and generally understand at least a couple of hundred more. This expansion in vocabulary showcases cognitive abilities, particularly pertaining to language acquisition and comprehension. While aspects like motor skills, social interactions, and emotional development are essential parts of a toddler's development, they do not directly relate to the ability to differentiate between known and unknown words. Language development is a cognitive achievement that reflects an infant's expanding understanding of their environment and communication skills.