Final answer:
The correct answer is c. their ability to make food, which distinguishes heterotrophic animals from autotrophic plants in classification.
Step-by-step explanation:
The characteristic used to place organisms into kingdoms is c. their ability to make food. This criterion differentiates heterotrophic organisms, like those in the animal kingdom, from autotrophic organisms, such as most plants that are capable of photosynthesis. Animals cannot make their own food and must obtain nutrients by eating other living things. In contrast, plants produce their own nutrients through photosynthesis. Therefore, the way an organism obtains its energy is a fundamental trait used in the classification of life forms into different kingdoms.