The Montreal Protocol refers to:

Options

A. persistent organic pollutants
B. global warming and climate change
C. substances that deplete the ozone layer
D. biosafety of genetically modified organisms

Explanation

**Step-by-Step Solution:**

1. **Understand the Montreal Protocol**: The Montreal Protocol is an international treaty that was designed to protect the ozone layer by phasing out the production and consumption of substances that deplete it.

2. **Identify the Year of Signing**: The Montreal Protocol was signed in 1987 and finalized in 1989 under the auspices of the United Nations.

3. **Recognize the Purpose**: The main purpose of the Montreal Protocol is to provide rules and guidelines for countries to phase out substances that harm the ozone layer, such as chlorofluorocarbons (CFCs).

4. **Evaluate the Options**:
- **Persistent Organic Pollutants**: This is related to the Stockholm Convention, not the Montreal Protocol.
- **Global Warming and Climate Change**: This is addressed by the Kyoto Protocol, not the Montreal Protocol.
- **Substances that Deplete the Ozone Layer**: This is the correct focus of the Montreal Protocol.
- **Biosafety of Genetically Modified Organisms**: This is covered by the Cartagena Protocol, not the Montreal Protocol.

5. **Select the Correct Option**: Based on the evaluation, the correct answer is that the Montreal Protocol refers to "substances that deplete the ozone layer."

**Final Answer**: The Montreal Protocol refers to substances that deplete the ozone layer.

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