## Core Concept
The age at which one becomes a major, or legally an adult, varies by country and jurisdiction but is commonly referred to in the context of Indian law. In India, the legal age of majority is defined by the Indian Majority Act.

## Why the Correct Answer is Right
The Indian Majority Act, 1875, specifies that a person attains majority on completing 18 years of age. This is a critical age milestone because it marks the transition from minority to adulthood, bringing with it full legal rights and responsibilities.

## Why Each Wrong Option is Incorrect
- **Option A:** 21 years. This was indeed the age of majority prior to the amendment of the Indian Majority Act in 1956, when it was reduced to 18 years. However, it is no longer the correct age of majority.
- **Option B:** 16 years. This age is relevant in certain contexts, such as the legal age for leaving school, but it is not the age of majority.
- **Option D:** 25 years. This is not a recognized age of majority or any significant legal milestone in the Indian context.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the age of majority in India is **18 years**, as per the Indian Majority Act, 1875. This is crucial not only for legal purposes but also for understanding patient autonomy in medical practice.

## Correct Answer: C. 18 years.