Which one of the following can fix nitrogen is
(a) Isogamy
(b) Anisogamy
(c) Syngamy
(d) None of the above

Options

A. Isogamy
B. Anisogamy
C. Syngamy
D. None of the above.

Answer

To solve the question "Which one of the following can fix nitrogen?", we need to analyze each of the given options:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is a process where nitrogen (N2) from the atmosphere is converted into ammonia (NH3) or related compounds in soil and water. This process is primarily carried out by certain bacteria and archaea, not by the reproductive processes mentioned in the options.

2. **Analyzing the Options**:
- **(a) Isogamy**: This term refers to a type of sexual reproduction where gametes are similar in size and shape. It does not involve nitrogen fixation.
- **(b) Anisogamy**: This refers to a type of sexual reproduction involving dissimilar gametes (usually a larger female gamete and a smaller male gamete). This process also does not relate to nitrogen fixation.
- **(c) Syngamy**: This term describes the fusion of male and female gametes to form a zygote. Like the previous two options, it does not pertain to nitrogen fixation.
- **(d) None of the above**: Since none of the first three options are associated with nitrogen fixation, this option suggests that none of the listed processes can fix nitrogen.

3. **Conclusion**:
- Since isogamy, anisogamy, and syngamy are all modes of sexual reproduction and do not involve nitrogen fixation, the correct answer is **(d) None of the above**.

### Final Answer:
**(d) None of the above**