Final answer:
Charlie has acquired the concept of gender constancy, which means understanding that one's gender is persistent despite changes in activities, appearances, or roles.
Step-by-step explanation:
Six-year-old Charlie realizes that his sister remains a girl even when she operates a bulldozer. Charlie has acquired the concept of gender constancy. Gender constancy is the understanding that one's gender remains the same even when external activities, appearances, or roles change, and this acquisition is a key stage in gender development for young children. Societal roles and expectations often perpetuate the division of gender norms, leading to specific activities being labeled as masculine or feminine. These gender stereotypes can shape the play choices of children, where boys may be encouraged to engage with trucks and toy guns, while girls might be given dolls to play with. However, when a child like Charlie recognizes that his sister's gender does not change when taking on a role traditionally seen as masculine, it shows a cognitive development in how he perceives gender beyond societal stereotypes.