The correct answer is option 'c' - The Lok Sabha.

The Lok Sabha, also known as the lower house of the Parliament of India, exercises greater power over the Union budget compared to the other institutions mentioned.

1. Role of the Lok Sabha in the budgetary process:
- The Lok Sabha has the primary responsibility of approving and passing the Union budget. The budget is presented by the Finance Minister in the Lok Sabha, and it undergoes a detailed examination and discussion by the Members of Parliament (MPs) representing various constituencies.
- The Lok Sabha has the power to make amendments and propose changes to the budget. It can discuss and debate the allocations and expenditures proposed by the government, and suggest modifications to ensure better utilization of funds.
- The Lok Sabha also has the authority to reject the budget if it is dissatisfied with the government's proposals. This can lead to a rethinking and revision of the budget by the government to better align with the interests and demands of the MPs.

2. Importance of the Lok Sabha in the budgetary process:
- The Lok Sabha represents the direct voice and aspirations of the people as it consists of elected representatives. The MPs are accountable to the people and are expected to raise and address the concerns of their constituents during budget discussions.
- The Lok Sabha reflects the diversity of the country and ensures that the budget caters to the needs of various regions, communities, and sectors. MPs can advocate for the allocation of funds to specific projects and developmental initiatives in their constituencies.
- The Lok Sabha's approval of the budget is necessary to authorize the government to spend money on various programs and activities. Without the approval of the Lok Sabha, the government cannot implement its proposed budget.

In contrast, the other institutions mentioned do not exercise as much power over the Union budget:

- The Reserve Bank of India (RBI) is the central bank of the country and primarily focuses on monetary policy and regulation of the financial system. While it may provide inputs and recommendations on fiscal matters, the final decision-making authority lies with the government and the Parliament.
- The Rajya Sabha, or the upper house of Parliament, also has a role in the budgetary process. However, it cannot reject or make changes to the budget. It can only suggest amendments, which may or may not be accepted by the Lok Sabha.
- The Supreme Court of India's role is to interpret and uphold the Constitution and ensure the legality and constitutionality of actions taken by the government. While it can intervene in matters related to budgetary allocations if they are found to be unconstitutional, its power over the budget is limited compared to the legislative bodies.

In summary, the Lok Sabha exercises greater power over the Union budget due to its role in approving, amending, and rejecting the budget proposals. It represents the voice of the people and plays a crucial role in ensuring that the budget meets the needs and aspirations of the country.