Explanation:

Regarding the Cornwallis System and the establishment of policing in colonial India:

1. Under the Cornwallis System, districts were divided into thanas or police jurisdictions of 20-30 miles square. - This statement is correct.

2. Each of them was placed under an officer of government, under the denomination of daroga. - This statement is correct.

3. The daroga system was introduced in Madras in 1812. - This statement is incorrect as the daroga system was initialized in Bengal, not specifically noted as being introduced in Madras in 1812.

4. The daroga was nominated by the district magistrate. - This statement is correct.

Option 4 (1, 2 and 4) is the correct choice