Final answer:
The answer to the question regarding Piaget's stage of concrete operations is 'C. Conservation', which signifies an understanding that quantities remain consistent despite changes in shape or appearance.
Step-by-step explanation:
Piaget's stage of concrete operations is a key concept in cognitive development theory, referring to the period when children between the ages of 7 to 11 start to think logically about concrete events and can grasp the concept of conservation. This means that children understand that even if the appearance of something changes, its amount, mass, or volume remains the same, provided nothing is added or removed. Therefore, the option that includes Piaget's stage of concrete operations is 'C. Conservation'