When 18-month-old James is given peas for the first time, he picks one up, throws it, and says "ball." According to Piaget’s theory, James is most likely the pea into his ball scheme.

Assimilating.

Explanation:

Jean Piaget developed a theory of Cognitive Development in the 1920s. In his theory, he claimed that children develop mentally by going through four stages.

In these four stages, one important aspect of the development is assimilation. Assimilation is a cognitive process in which children form new schemas out of the existing ones.

In the given case, James, an 18-month-old child, calls peas a ball due to the assimilating process.

Therefore, assimilation is the correct answer.