A colourless parasitic red algae is

Options

A. Harveyella
B. Batrachospermum
C. Porphyra (red alga)
D. Cephaleuros

Answer

To solve the question "A colourless parasitic red algae is?", we will analyze the options provided and determine which one fits the description.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a specific type of algae that is colorless, parasitic, and belongs to the red algae group (Rhodophyta).

2. **Analyzing the Options**:
- **Option A: Hervella**: This is known to be a colorless parasitic red algae. It is indeed a member of the red algae group and has characteristics of being parasitic.

- **Option B: Batrachospermum**: This is a type of red algae that is found in freshwater. It is not known to be parasitic and has a color that varies from violet to blue-green.

- **Option C: Porphyra**: This is also a red algae, commonly found in cold waters and is not parasitic. It is typically greenish in color and is used in food products like nori.

- **Option D: Cephalorus**: This is a genus of green algae and is not classified as red algae. It is also parasitic but does not fit the criteria of being a red algae.

3. **Conclusion**: After analyzing all the options, the only one that fits the description of being a colorless parasitic red algae is **Option A: Hervella**.

### Final Answer:
The correct answer is **Hervella**.

---