Unicellular cyanobacteria reproduce asexually by
(a) Binary fission
(b) Fragmentation
(c) Hormogones
(d) Conjugation

Options

A. Binary fission
B. Fragmentation
C. Hormogones
D. Conjugation.

Answer

To solve the question regarding how unicellular cyanobacteria reproduce asexually, we can analyze each option provided:

1. **Understanding the question**: The question asks for the method of asexual reproduction in unicellular cyanobacteria.

2. **Evaluating the options**:
- **(a) Binary fission**: This is a common method of asexual reproduction in unicellular organisms, including cyanobacteria. In binary fission, a single cell divides into two identical daughter cells.
- **(b) Fragmentation**: This method involves breaking into fragments, which can develop into new individuals. However, fragmentation is more common in colonial or filamentous forms of cyanobacteria rather than unicellular forms.
- **(c) Hormogones**: Hormogones are motile filaments that can be produced by some cyanobacteria, but they are not a method of asexual reproduction. They are more related to movement and dispersal.
- **(d) Conjugation**: This is a form of sexual reproduction where genetic material is exchanged between two organisms. It is not typical for unicellular cyanobacteria, as they reproduce asexually.

3. **Conclusion**: Based on the analysis, the correct method of asexual reproduction for unicellular cyanobacteria is **(a) Binary fission**.

### Final Answer:
**(a) Binary fission**