Five-year-old Lance complained to his mom that the pizza was not big enough. When she cut the pizza into smaller slices, Lance was happy because he now thought that there was more pizza. Lance lacks the concept of:

The answer is conservation.

Explanation:

The concept of conservation explains that an object will preserve its original properties even if the parts are differently shaped or distributed. In the example, Lance does not understand that the amount of pizza stays the same when cut into different slices.

According to Piaget, understanding of conservation appears during the concrete operational stage, around age seven.