**Core Concept**
The Infant Milk Substitutes Act (IMS) is a legislation aimed at regulating the production, distribution, and marketing of infant milk substitutes, such as formula milk, to ensure their safety and quality for infants.

**Why the Correct Answer is Right**
The IMS was enacted to prevent the promotion and distribution of substandard or misleading infant milk substitutes, which can lead to malnutrition and other health problems in infants. The act provides guidelines for the labeling, packaging, and advertising of infant milk substitutes, as well as regulations for the establishment and operation of infant milk substitute factories. The IMS is a crucial piece of legislation that protects the health and well-being of infants in India.

**Why Each Wrong Option is Incorrect**
**Option A:** The IMS was not enacted in 1975, as this is the year the IMC (Indian Medical Council) Act was passed.
**Option B:** The IMS is not related to the Food Safety and Standards Act, which is a separate legislation that regulates food products in India.
**Option C:** The IMS was not enacted in 1998, as this is the year the National AIDS Control Organization (NACO) was established.

**Clinical Pearl / High-Yield Fact**
The IMS is an example of a legislation that regulates the marketing of infant milk substitutes, which is a critical aspect of infant health and nutrition. It is essential for healthcare professionals to be aware of the IMS and its guidelines to provide accurate information to parents and caregivers.

**Correct Answer:** D. 1981.