Explanation:

- The Government of National Capital Territory of Delhi (Amendment) Bill, 2021 aims to redefine the governance structure of Delhi.

- It amends the Government of National Capital Territory of Delhi Act, which dates back to a specific year.

- Option 1 - 1998: Incorrect. The Act in question is not from this year.

- Option 2 - 1994: Incorrect. There is no such Act from this year affecting Delhi's governance in this context.

- Option 3 - 1991: Incorrect due to duplication error.

- Option 4 - 1991: Correct. The original Act was enacted in 1991.