Step 1: Understanding the question:
The question asks for the year India's Parliament enacted the law criminalising the practice of untouchability.
Step 2: Key fact:
The Untouchability (Offences) Act was passed in 1955, making the practice of untouchability in any form a punishable offence; it was later renamed and strengthened as the Protection of Civil Rights Act, 1955 (amended in 1976).
Step 3: Why other options are wrong:
1947 is India's Independence year, not the year of this specific law; 1950 is when the Constitution (which itself abolished untouchability under Article 17) came into force, a related but distinct milestone; 1957 does not correspond to this Act.
Final Answer:
1955 (option iv).