Blue pigment is

Options

A. Phycocyanin
B. Phycoerythrin
C. Plastocyanin
D. `P_(680)`

Answer

To answer the question "Blue pigment is," we need to analyze the options provided and identify which one corresponds to the blue pigment found in blue-green algae or cyanobacteria.

### Step-by-Step Solution:

1. **Understanding the Context**: The question is asking about a blue pigment, specifically in relation to blue-green algae (cyanobacteria).

2. **Identifying the Options**: The options provided are:
- A. Phycocyanin
- B. Phycoerythrin
- C. Plastocyanin
- D. P680

3. **Analyzing Each Option**:
- **Option A: Phycocyanin**: This is known to be a blue pigment found in cyanobacteria. It is responsible for the blue color in these organisms.
- **Option B: Phycoerythrin**: This is a red pigment, not blue. It is also found in some algae but does not contribute to the blue color.
- **Option C: Plastocyanin**: This is a protein involved in electron transfer in photosynthesis, not a pigment. Therefore, it cannot be the answer.
- **Option D: P680**: This refers to a specific wavelength (680 nm) associated with Photosystem II in photosynthesis, but it is not a pigment itself.

4. **Conclusion**: Based on the analysis, the blue pigment in question is Phycocyanin. Therefore, the correct answer is:
- **A. Phycocyanin**

### Final Answer:
The blue pigment is **Phycocyanin**.