The head of the National Defence Committee is the Prime Minister.

Explanation:
The National Defence Committee is a committee that plays a crucial role in the defense and security of a country. In the case of India, the head of the National Defence Committee is the Prime Minister.

Here is a detailed explanation of why the Prime Minister is the head of the National Defence Committee:

1. Role of the National Defence Committee:
- The National Defence Committee is responsible for formulating and implementing defense policies and strategies.
- It oversees the defense forces and ensures the country's security and territorial integrity.
- The committee also reviews defense budgets, procurement decisions, and evaluates defense preparedness.

2. Prime Minister as the Head:
- The Prime Minister is the highest-ranking political leader in the country and holds significant executive powers.
- As the head of the government, the Prime Minister is responsible for making key decisions related to defense and national security.
- The Prime Minister is also the Chairperson of the Cabinet Committee on Security (CCS), which is the highest decision-making body on defense and security matters.
- The CCS is a part of the National Defence Committee and advises the government on matters of defense policy and strategy.

3. Prime Minister's Role in Defense:
- The Prime Minister is responsible for setting the overall defense policy of the country.
- He/she takes into consideration various factors such as national security threats, geopolitical situation, and defense capabilities.
- The Prime Minister also provides strategic guidance to the armed forces and ensures their readiness.
- In times of crisis or conflict, the Prime Minister takes critical decisions related to defense operations and national security.

In conclusion, the Prime Minister is the head of the National Defence Committee because of his/her role as the highest-ranking political leader, executive powers, and responsibility for setting the defense policy of the country.