The correct answer is option 'D', i.e. 'Equality'.

Explanation:

Untouchability refers to the social practice of discriminating people based on their caste or social status. It is a form of caste-based discrimination that has been prevalent in India for centuries. The Constitution of India, which was adopted in 1950, abolished untouchability and declared it a punishable offence.

The prohibition of untouchability comes under the concept of 'Equality', which is one of the fundamental principles of the Indian Constitution. The idea of equality implies that all individuals are equal before the law and are entitled to equal protection and equal opportunities. It prohibits any kind of discrimination based on caste, creed, religion, gender, or any other grounds.

The prohibition of untouchability is based on the principle of equality, as it seeks to eliminate the discriminatory practice of treating certain groups of people as inferior or untouchable. It ensures that every individual, irrespective of their caste or social status, is treated with dignity and respect.

Conclusion:

To conclude, the prohibition of untouchability comes under the concept of 'Equality', which is one of the fundamental principles of the Indian Constitution. The Constitution seeks to eliminate all forms of discrimination and ensure that every individual is treated with equal dignity and respect. The prohibition of untouchability is a step towards achieving this goal.