In the Indian Constitution, the concept of Welfare State is described in the Directive Principles of State Policy (option 2). The Directive Principles of State Policy are guidelines and principles for the government to follow while making laws and policies. These principles are not enforceable in court but are considered fundamental in the governance of the country.

Option 1, the Preamble, outlines the basic ideals and objectives of the Constitution, including justice, liberty, equality, and fraternity, but it does not specifically mention the concept of a Welfare State.

Option 3, Fundamental Rights, provide individuals with certain basic rights and liberties, but they do not explicitly address the concept of a Welfare State.

Option 4, Fundamental Duties, are a set of moral obligations for citizens but do not directly relate to the concept of a Welfare State.

In conclusion, the correct option is 2, Directive Principles of State Policy, as it includes provisions for the government to work towards securing the welfare of the people by implementing social and economic rights.