Dave has a ball of Silly Putty. His 6-year-old son, Mason, watches as Dave flattens the Silly Putty into a thin "pancake." When Dave asks Mason if there is now more Silly Putty, Mason replies "Yes" because he has:

not mastered the concept of conservation

Explanation:

Conservation is the understanding that something stays the same in quantity even though its appearance changes. To be more technical conservation is the ability to understand that redistributing material does not affect its mass, number, volume or length.

The concrete operational stage is the third in Piaget's theory of cognitive development. This stage lasts around seven to eleven years of age, and is characterized by the development of organized and rationale thinking.Children gain the abilities of conservation (number, area, volume, orientation), reversibility, transitivity and class inclusion.

Since mason is still 6 years old he has not gotten to the concrete operational stage of 7-11 years old where he understand the conservation of the ball of silly putty into a thin pancake,hence he replied YES.