Fairly decomposed organic matter is called litter /detritus /duff.

Explanation

To answer the question "Fairly decomposed organic matter is called litter / detritus / duff," we need to analyze the definitions of each term provided in the question.

### Step-by-Step Solution:

1. **Understanding the Terms**:
- **Litter**: This refers to waste materials that can include a variety of organic and inorganic substances. It is not specifically decomposed and can include items like paper, plastic, and vegetable waste.
- **Detritus**: This term describes dead and decayed organic matter. It indicates that the organic material has undergone some decomposition but may not be fully decomposed.
- **Duff**: This is a term used for organic matter that has undergone significant decomposition. It refers to the layer of decomposed organic material found on the forest floor, which is fairly decomposed.
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