One of the instruments of selective credit control in India that is not mentioned in the list is the Variable Cash Reserve Ratio (CRR).

The Cash Reserve Ratio is the percentage of total deposits that banks are required to keep with the central bank as a reserve. It is a tool used by the central bank to control the money supply in the economy. When the central bank increases the CRR, it reduces the amount of funds available for lending by commercial banks, thereby reducing the money supply. Conversely, when the central bank decreases the CRR, it increases the amount of funds available for lending, which stimulates economic growth.

The Variable Cash Reserve Ratio refers to the practice of adjusting the CRR as a tool of selective credit control. It allows the central bank to increase or decrease the CRR based on the prevailing economic conditions and monetary policy objectives. By varying the CRR, the central bank can influence the liquidity in the banking system and control inflation or stimulate economic growth.

The other options mentioned in the list are instruments of selective credit control in India:

a) Regulation of consumer credit: This refers to the central bank's ability to regulate the amount of credit that can be extended to consumers. By setting limits on consumer credit, the central bank can prevent excessive borrowing and control inflationary pressures.

b) Rationing of credit: This refers to the central bank's ability to allocate credit to different sectors of the economy based on their priority. By rationing credit, the central bank can ensure that essential sectors receive adequate funding while limiting credit to non-priority sectors.

c) Margin requirements: This refers to the central bank's ability to regulate the amount of collateral that borrowers need to provide when taking a loan. By increasing margin requirements, the central bank can reduce the amount of credit available for speculative purposes and control excessive risk-taking.

In summary, the Variable Cash Reserve Ratio is an instrument of selective credit control in India that allows the central bank to adjust the CRR based on economic conditions. The other options mentioned in the list are also instruments of selective credit control in India.