What did Piaget mean by an "operation" (as in "concrete operations") ?

A) an organized way of dealing with objects in the world
B) applying a schema to a new object
C) modifying a schema to fit a new object
D) a mental process that can be reversed

Piaget defined an operation as a mental process that can be reversed. This means that a child can mentally undo an action and return an object to its original state. In Piaget's theory, concrete operations are the mental processes that allow children to think logically about concrete objects and events in the real world. These operations involve the ability to classify objects, understand relationships between objects, and perform mental transformations.