Explanation:

- Option 1: Carbon tax - Correct, a cess on coal at Rs 100 per ton is a form of carbon tax. It is levied to discourage the use of coal by making it more expensive due to its carbon emissions.

- Option 2: Carbon subsidy - This term refers to financial support to reduce emissions, not applicable here.

- Option 3: Carbon incentive for technology - This would involve rewards or support for technological advancements, not a tax.

- Option 4: Carbon incentive for selling carbon permit - Related to trading systems, not a tax imposed per ton of coal.