Infants develop a fear of strangers after about 8 months of age because they can't assimilate unfamiliar faces into their

The correct answer is schemas.
Given that infants cannot assimilate unfamiliar faces into their schemas, they are likely to exhibit fear of strangers at the age of around 8 months. Schemas refer to patterns or models of something. So the infant has a schema of all the people it already knows, such as its parents, but when it meets someone new who doesn't fit into that schema, it may become afraid of them.