One carbon credit represents the right to emit one ton of carbon dioxide or an equivalent amount of a different greenhouse gas. These credits can be traded in the carbon market as part of initiatives to reduce overall greenhouse gas emissions.

The concept of carbon credits was indeed ratified in conjunction with the Kyoto Protocol, so option A is correct.

The goal of the carbon credit system is indeed to limit the increase of carbon dioxide emission, making option C correct.

Carbon credits are indeed awarded to countries or groups that have reduced greenhouse gases below their emission quota, so option D is also correct.

However, option B is not correct. The price of carbon credits is not fixed by the United Nations Environment Programme. Instead, the price of carbon credits is determined by the market, based on supply and demand.