Name the single-celled marine alga which is about 10 cm long.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the name of a single-celled marine alga that is approximately 10 cm long.

2. **Identifying Key Characteristics**: The alga is described as:
- Single-celled (unicellular)
- Marine (found in oceanic environments)
- Approximately 10 cm in length

3. **Recalling Relevant Information**: Based on the characteristics provided, we need to think about unicellular marine algae that can grow to about 10 cm.

4. **Identifying the Alga**: The alga that fits this description is called **Acetabularia**. It is known for its unique structure and size, reaching up to 10 cm in length.

5. **Additional Information**: Acetabularia is also commonly referred to as the "umbrella plant" due to its shape. It belongs to the family Polyphysaceae.

6. **Final Answer**: Therefore, the single-celled marine alga that is about 10 cm long is **Acetabularia**.

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