— is a mixture of gases given off when animal and plant matter rots in the absence of air.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the name of a mixture of gases that is produced when animal and plant matter decomposes in the absence of air.

2. **Identifying the Process**: The process described is decomposition, which occurs when organic matter (like animal and plant matter) breaks down. This process can happen in the presence of air (aerobic) or in the absence of air (anaerobic).

3. **Recognizing the Conditions**: The key condition in the question is that this decomposition occurs in the absence of air. This anaerobic condition is crucial for the production of certain gases.

...