Viruses parasitising bacteria are

Options

A. Bacteriophages
B. Bacto-viruses
C. Retroviruses
D. Plasmids.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about viruses that specifically parasitize bacteria. This means we are looking for a type of virus that infects and harms bacterial cells.

2. **Identifying the Options**: The options provided are:
- A) Bacteriophage
- B) Bactovirus
- C) Retroviruses
- D) Plasmids

3. **Analyzing Each Option**:
- **A) Bacteriophage**: This is a type of virus that specifically targets and infects bacteria. Bacteriophages attach to bacterial cells, inject their genetic material, and often lead to the destruction of the bacterial cell. This matches the description of a virus that parasitizes bacteria.

- **B) Bactovirus**: This term is not commonly recognized in virology. There is no established virus known as "bactovirus," making this option incorrect.

- **C) Retroviruses**: Retroviruses are a type of virus that primarily infects eukaryotic cells and has RNA as their genetic material. They convert their RNA into DNA once inside the host cell. They do not specifically target bacteria, so this option is incorrect.

- **D) Plasmids**: Plasmids are small, circular pieces of DNA found in bacteria, which can carry genes for antibiotic resistance. They are not viruses and do not parasitize bacteria, making this option incorrect.

4. **Conclusion**: After analyzing all the options, the only correct answer is **A) Bacteriophage**, as these viruses are known for infecting and parasitizing bacteria.

**Final Answer**: Viruses parasitizing bacteria are **Bacteriophages**.

---