The Chairman of the Defence Planning Committee, set up in April 2018, is the National Security Advisor. The Defence Planning Committee was created to provide integrated planning and execution of defense-related matters.

Now, let`s analyze the other options:

Option 1 states that the Prime Minister is the Chairman of the Defence Planning Committee, which is incorrect. The Prime Minister is the head of the government and has various other responsibilities, but this particular committee is chaired by the National Security Advisor.

Option 3 suggests that the Defence Minister is the Chairman of the committee. While the Defence Minister plays a crucial role in defense-related matters, the Chairman of the Defence Planning Committee is the National Security Advisor, not the Defence Minister.

Option 4 states that the Chief of the Army Staff is the Chairman. However, the Chief of the Army Staff is a position within the military and does not hold the responsibility of chairing the Defence Planning Committee.

In conclusion, the correct answer is option 2, the National Security Advisor.