Detailed Solution:
The correct answer is “the Directive Principles of the State Policy”.
While the Preamble does outline the vision of India as a sovereign, socialist, secular, democratic republic, it doesn't explicitly mention the concept of a welfare state.
The Directive Principles of State Policy explicitly outline the state's responsibility to work towards the welfare of its citizens. These principles, though not enforceable by courts, guide the state's actions towards building a socialistic pattern of society and ensuring the well-being of its people.