**Core Concept**
The question requires knowledge of significant legislative milestones in India, specifically Acts passed before 1980 that impact public health, labor rights, and social welfare.

**Why the Correct Answer is Right**
Among the given options, the Factories Act (1948) is a crucial legislation that aimed to regulate working conditions, health, and safety in industrial settings. This Act was enacted before 1980 and has been a cornerstone in protecting workers' rights in India. The Factories Act mandated the provision of safe working conditions, regular working hours, and adequate compensation for workers.

**Why Each Wrong Option is Incorrect**
**Option A:** The Employees' State Insurance (ESI) Act was passed in 1948, but it was amended in 2010 to increase the coverage and benefits for insured persons. However, the primary focus of the ESI Act is not on regulation of working conditions but on providing health insurance.

**Option B:** The Pre-Natal Diagnostic Techniques (PNDT) Act was enacted in 1994 to prevent sex-selective abortion and regulate prenatal diagnostics. This Act was passed after 1980.

**Option C:** The Air (Prevention and Control of Pollution) Act was passed in 1981, with the primary objective of controlling air pollution from various sources.

**Option D:** The Medical Termination of Pregnancy (MTP) Act was enacted in 1971, but it was amended in 2021 to increase the gestation limit for abortion. Although this Act is relevant to public health, it was passed after 1980.

**Clinical Pearl / High-Yield Fact**
Students should remember that various Acts and regulations have been enacted to protect workers' rights and public health in India. Understanding the historical context and significance of these Acts is essential for public health professionals and policymakers.

**Correct Answer:** C. Factories act