The correct answer is option 3: through a cabinet decision in this regard.

The Planning Commission of India was constituted through a cabinet decision, meaning that the decision to establish the Planning Commission was made by the cabinet of ministers. This decision was not based on a specific provision mentioned in the constitution, nor was it established through an Act of Parliament or a constitutional amendment.

It is important to note that the Planning Commission of India was a government agency that was responsible for formulating and implementing five-year plans for the socio-economic development of the country. It played a crucial role in the planning and allocation of resources for various sectors and projects in India.

Although option 1 mentions a constitutional provision with specific mention for the Planning Commission, it is not the correct answer as the Planning Commission was not established under any constitutional provision. Similarly, options 2 and 4 are not correct as the Planning Commission was not established through an Act of Parliament or a constitutional amendment.

Therefore, the correct answer is option 3.