An 8-year-old child is unable to identify the plate with a greater number of chocolates when a second plate with a different amount of chocolates is placed in front of him or her. which stage of cognitive development has the child failed to attain

The child failed to attain the concrete operational stage of  cognitive development. 

The concrete operational stage of development is a stage of cognitive development that usually occurs during ages 7-11 in children. A key component of this stage is 'conservation'- that is, the ability to realize that the quantity of objects does not change despite their arrangement or physical appearance.

In this instance, the child is unable to identify which plate has a greater number of chocolates because she has not developed the concept of conservation and thus, has failed to attain the concrete operational stage of cognitive development.