The Santal Parganas Tenancy Act, 1949 is a significant piece of legislation aimed at protecting the rights of tenants in the Santal Parganas region. Understanding the role of various authorities in the enforcement of this act is crucial for administrative and governance aspects.

The question asks which authority plays the most decisive role in enforcing the Santal Parganas Tenancy Act, 1949. Let's analyze each option:

1. Gram Sabha: While the Gram Sabha plays an essential role in the local self-government system, especially in villages, it generally doesn't have the decisive legal authority needed for enforcing tenancy laws at the level required by this act.
2. Deputy Commissioner: The Deputy Commissioner is a senior administrative officer in a district and often has the authority to enforce land and tenancy laws. In the context of the Santal Parganas Tenancy Act, 1949, the Deputy Commissioner is granted significant powers for enforcement, overseeing, and addressing disputes under this act. This makes the Deputy Commissioner the most decisive authority in this context.
3. State Legislature: Although the State Legislature is responsible for creating laws, it doesn't directly enforce them. Enforcement is typically the role of administrative and judicial bodies.
4. Civil Courts: Civil Courts can adjudicate disputes but do not have the same immediate enforcement role as an administrative authority like the Deputy Commissioner would.

Based on the above analysis, the Deputy Commissioner is identified as the authority that plays the most decisive role in the enforcement of the Santal Parganas Tenancy Act, 1949. This is because the Deputy Commissioner is endowed with significant statutory powers to implement and manage tenancy laws within the district effectively.