Final answer:
Children in the concrete operational stage cannot conserve length.
Step-by-step explanation:
The ability that children cannot do during the concrete operational stage, as evident by Piaget's conservation experiment, is conservation of length. In the experiment, children are asked to compare the length of two objects before and after a transformation. Children in the concrete operational stage can conserve number, mass, volume, but not length. They have difficulty understanding that the length of an object remains the same even if it is stretched or compressed.