Which one of the following categories of Fundamental Rights/incorporates protection against untouchability as a form of discrimination?

Untouchability is a form of discrimination that violates the principle of equality among citizens, regardless of caste. To uphold this principle, Article 17 of the Constitution explicitly abolishes untouchability, making it one of the five rights under the broader Right to Equality (Articles 14-18). Therefore, the correct answer is option D.