**Core Concept**
Carbon credits are a market-based approach to reducing greenhouse gas emissions, where one ton of CO2 equivalent emissions is equivalent to one CER. This concept is based on the principle of carbon offsetting, where emissions reductions in one region are used to compensate for emissions in another.

**Why the Correct Answer is Right**
Each CER is equivalent to one ton of CO2 equivalent emissions reduced or avoided. This is calculated based on the amount of greenhouse gases emitted, taking into account their global warming potential. The CER is issued to represent the verified emission reductions, which can be traded on the carbon market.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CERs are not equivalent to a specific quantity of metric tons of CO2, but rather to the reduction or avoidance of one ton of CO2 equivalent emissions.
**Option B:** This option is incorrect because CERs are not equivalent to a specific quantity of metric tons of CO2, but rather to the reduction or avoidance of one ton of CO2 equivalent emissions.
**Option C:** This option is incorrect because CERs are not equivalent to a specific quantity of metric tons of CO2, but rather to the reduction or avoidance of one ton of CO2 equivalent emissions.

**Clinical Pearl / High-Yield Fact**
Carbon credits can be used to finance projects that reduce greenhouse gas emissions, such as renewable energy projects or reforestation efforts. This can help to mitigate climate change and promote sustainable development.

**Correct Answer: D. One ton of CO2 equivalent emissions.**