Explanation:

- The statement, "when a class system becomes closed to vertical mobility, it becomes a caste," focuses on how social mobility differentiates class and caste systems.

- Option 1: Hiller

- Hiller is credited with this observation.

- He said a class system changes into a caste system if there’s no vertical mobility.

- Option 2: Miller

- Miller is not generally linked to this specific statement about class and caste.

- Option 3: Dahrendorf

- Dahrendorf’s theories focus more on authority and class conflict, not specifically on this point.

- Option 4: Karl Marx

- Marx differentiated between classes mainly around ownership of production, but did not focus on caste as a closed system.

Correct Answer: Option 1 - Hiller