The most obvious change as children move from the sensorimotor to the preoperational stage is an increase in

symbolic activity

Explanation:

A Swiss developmental psychologists named Jean Piaget propounded the theory of cognitive development, which explains how human intelligence and knowledge is being developed and used as an individual progresses from infancy through adulthood.  

Piaget’s theory of development describes this cognitive development of the human intelligence in 4 stages which are: sensorimotor, preoperational, concrete operational and formal operational stage.

The first stage is the sensorimotor stage is observed from birth through ages 18-24 months. At this stage, the child's cognitive development increases as he interacts with his environment and learns how to crawl, stand, and walk.  

As children move to the second stage which is the preoperational stage, they begin to develop some symbolic activities such as early language development, which increases at the end of this stage.  

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