Option 1: 1947 - This is not the correct answer. The decimal system of currency was not introduced in India in 1947.

Option 2: 1952 - This is not the correct answer. The decimal system of currency was not introduced in India in 1952.

Option 3: 1950 - This is not the correct answer. The decimal system of currency was not introduced in India in 1950.

Option 4: 1957 - This is the correct answer. The decimal system of currency was introduced in India in 1957. Before this, India used the British system, which was based on the rupee, which was further divided into 16 annas or 64 paise. The decimal system replaced this complex system and introduced the rupee as the basic unit of currency, with 100 paise making up one rupee. This change was made to simplify calculations and make transactions easier for the Indian population.