Explanation:

- When an untouchable converts to Christianity, their social status in the caste system changes significantly.

- Option 1: Horizontal mobility – This is a movement within the same social level or class, like changing jobs but remaining in the same class.

- Option 2: Vertical mobility – This is a movement up or down the social hierarchy, like moving from a lower to a higher class or vice versa.

- Option 3: Social mobility – This is a broad term that includes both horizontal and vertical mobility.

- Option 4: None of the above – Means the change doesn't fit any kind of social mobility.

Correct Answer:

Option 2: Vertical mobility

- Converting from being an "untouchable" (very low social status) to Christianity (which doesn't recognize caste), is considered an upward movement in social hierarchy.