The correct answer is option 4: 1000 kg of carbon dioxide.

Carbon credits are a unit of measurement used in carbon trading to quantify and track greenhouse gas emissions. They are a way to reduce carbon emissions by assigning a monetary value to the amount of carbon dioxide (CO2) or other greenhouse gases that are being released into the atmosphere.

In this question, one carbon credit is accepted as equivalent to 1000 kg of carbon dioxide. This means that if a company or individual wants to offset their carbon emissions, they can purchase carbon credits to offset the amount of CO2 they are releasing. By purchasing these credits, they are essentially investing in projects and initiatives that reduce CO2 emissions elsewhere, such as renewable energy projects or reforestation efforts.

It is important to note that carbon credits are specific to carbon dioxide and not carbon itself. Carbon dioxide is a greenhouse gas that contributes to climate change, while carbon is a chemical element that can exist in various forms. Therefore, options 1 and 3, which mention carbon, are not correct. Option 2, which states 100 kg of carbon dioxide, is also incorrect.