A facultative parasite is one which :

Options

A. always requires dead organic matter
B. always requires a living host
C. is normally a parasite but can also become a saprophyte
D. normally a saprophyte but can also become a parasite

Explanation

To determine the correct definition of a facultative parasite, we can analyze the given options step by step.

### Step-by-Step Solution:

1. **Understanding the Definition of a Facultative Parasite**:
- A facultative parasite is an organism that typically lives as a saprophyte (feeding on dead organic matter) but has the ability to act as a parasite when conditions are unfavorable.

2. **Analyzing the Options**:
- **Option A**: "Always requires dead organic matter."
- This option describes saprophytic organisms, not facultative parasites. Therefore, this option is incorrect.

- **Option B**: "Always requires a living host."
- This option suggests that the organism cannot survive without a host, which contradicts the definition of facultative parasites. Thus, this option is also incorrect.

- **Option C**: "Normally a parasite but can also become a saprophyte."
- This option reverses the correct relationship. Facultative parasites are primarily saprophytic and can become parasitic under certain conditions. Therefore, this option is incorrect.

- **Option D**: "Normally a saprophyte but can also become a parasite."
- This option correctly describes facultative parasites, as they primarily feed on dead organic matter but can switch to parasitic behavior when necessary. Thus, this option is correct.

3. **Conclusion**:
- The correct answer is **Option D**: "Normally a saprophyte but can also become a parasite."