Facultative anaerobes :
(a) are photosynthetic
(b) do not always need oxygen
(c) are killed in an oxygenated environment
(d) require a constant supply of oxygen

Options

A. are photosynthetic
B. do not always need oxygen
C. are killed in an oxygenated environment
D. require a constant supply of oxygen

Explanation

To solve the question regarding facultative anaerobes, we need to analyze each option provided:

1. **Understanding Facultative Anaerobes**:
- Facultative anaerobes are organisms that can survive in both the presence and absence of oxygen. They can switch between aerobic respiration (when oxygen is available) and anaerobic respiration (when oxygen is not available).

2. **Evaluating Each Option**:
- **(a) Are photosynthetic**: This statement is incorrect. Facultative anaerobes are not necessarily photosynthetic; they can be heterotrophic or autotrophic but do not rely on photosynthesis for energy.
- **(b) Do not always need oxygen**: This statement is correct. Facultative anaerobes can survive without oxygen, which means they do not always need it.
- **(c) Are killed in an oxygenated environment**: This statement is incorrect. This characteristic applies to obligate anaerobes, which cannot survive in the presence of oxygen.
- **(d) Require a constant supply of oxygen**: This statement is also incorrect. Facultative anaerobes do not require a constant supply of oxygen; they can function without it.

3. **Conclusion**:
- The correct answer is **(b) do not always need oxygen**.

### Step-by-Step Solution:
1. Define what facultative anaerobes are.
2. Analyze option (a) and determine its validity.
3. Analyze option (b) and confirm its correctness.
4. Analyze option (c) and determine its validity.
5. Analyze option (d) and determine its validity.
6. Conclude which option is correct.