Explanation:

- Open Market Operation: The central bank buys or sells government securities in the open market to regulate the money supply. It's a key quantitative tool.

- Margin Requirements: Refers to the collateral required for loans. It's used to control the volume of credit but is a qualitative method, not quantitative.

- Variable Reserve Ratio: This involves changing the reserve requirements that banks must hold, affecting lending capacity. It's a quantitative method.

- Bank Rate Policy: Adjusting the interest rate at which the central bank lends to commercial banks. This influences overall credit conditions and is quantitative.

The correct answer is:

- Option 2: Margin Requirements