Final answer:
Dweck and her colleagues chose seventh graders for their study on motivation and learning because this age group is at a developmental stage ideal for examining changes in mindset and cognitive processes.
Step-by-step explanation:
Why Dweck and her colleagues decided to focus on seventh graders in their study is likely because this age group represents a specific developmental stage where students are transitioning from concrete operational thought to more formal operational thought, according to Piaget's theory of cognitive development. This phase is crucial for developing more complex thinking processes, including the capacity for abstract reasoning, which could be significantly influenced by motivation and mindset. Furthermore, seventh graders are at an age where they can understand and respond to the interventions in the study, making them ideal subjects for research related to learning and motivation.