"Carbon credits are traded at a price fixed from time to time by the United Nations Environment Programme."

The United Nations Environment Programme (UNEP) does not fix the price of carbon credits. Carbon credits are traded on the carbon market, where the price is determined by market forces of supply and demand. The price can vary depending on the type of carbon credit, its certification, and the prevailing market conditions.

Carbon credits are a part of a cap-and-trade system that was ratified in conjunction with the Kyoto Protocol. They are awarded to countries or groups that have reduced their greenhouse gas emissions below their emission quota. The goal of the carbon credit system is to limit the increase of carbon dioxide emissions and provide economic incentives to reduce greenhouse gas emissions.

Therefore, the correct statement is: "Carbon credits are not traded at a price fixed from time to time by the United Nations Environment Programme."