What is the ideological basis of untouchability?

Purity and impurity.

The correct answer is Option 1: Purity and impurity.

Option 1: Purity and impurity. Correct. The ideology of purity and pollution provides the basis for justifying untouchability by assigning ritual superiority or inferiority to people based on birth.

Option 2: Caste System. Incorrect. The caste system is the broader social institution within which untouchability operates. The ideological basis that justifies this practice is the belief in purity and pollution, not the caste system itself.

Option 3: Social Inequalities. Incorrect. Social inequalities are an outcome of practices such as untouchability, but they do not constitute its ideological basis.

Option 4: Social Stigma. Incorrect. Social stigma is a consequence of untouchability and the discrimination associated with it. It is not the belief system used to justify the practice.