Final answer:
Ben Stallworth's third-grade students, who are typically eight and nine years old, are likely in the concrete operational stage of cognitive development, where they can think logically about real events, understand conservation, and perform basic math operations.
Step-by-step explanation:
Third-grade teacher Ben Stallworth can expect that the majority of his eight- and nine-year-old students are in the concrete operational stage of cognitive development. According to Jean Piaget's theory, this stage occurs from about 7 to 11 years old. At this stage, children can think logically about tangible, real (concrete) events, understand the concept of conservation, and perform basic mathematical operations. They are also able to use memory strategies more effectively and appreciate that even if the appearance of something changes, its other properties like mass and volume remain consistent.