Explanation:

Let’s break down the options:

- Power: This matters in any social hierarchy, but it isn’t the core feature that makes caste “caste.” Other systems give power; caste adds something more.

- Service: Some people believe doing specific services (certain jobs) is how castes formed. But service alone doesn’t explain the system’s inner logic.

- Discrimination: Sure, caste is about discrimination. But this is more about the effect, not the original logic.

- Pollution: In caste, the idea of “purity” and “pollution” shaped everything—who could touch whom, eat together, intermarry, etc. This wasn’t just abstract. Everyday lives literally ran on this logic.

That’s why “pollution” is the right answer.