`RuBP` occurs in

Options

A. Krebs cycle
B. `C_(4)` plants
C. ETS
D. Calvin cycle.

Answer

### Step-by-Step Solution:

1. **Identify RuBP**: RuBP stands for ribulose bisphosphate, which is a 5-carbon sugar molecule that plays a crucial role in the process of photosynthesis.

2. **Understand its Role in Photosynthesis**: RuBP is involved in the fixation of carbon dioxide during the Calvin cycle, which is also known as the C3 cycle. This cycle is a part of the photosynthetic process in plants.

3. **Calvin Cycle Explanation**: In the Calvin cycle, RuBP reacts with carbon dioxide in the presence of the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase). This reaction leads to the formation of 3-phosphoglycerate (3-PGA), which is a 3-carbon compound.

4. **Differentiate Between C3 and C4 Plants**:
- In C3 plants, the first stable product formed after carbon fixation is a 3-carbon compound (3-PGA), hence the name C3 cycle.
- In contrast, C4 plants initially produce a 4-carbon compound (oxaloacetate, OAA) during carbon fixation, utilizing a different pathway involving the enzyme PEP carboxylase.

5. **Conclusion**: RuBP occurs in the Calvin cycle (C3 cycle) of photosynthesis, which is characteristic of C3 plants.

### Final Answer:
RuBP occurs in the Calvin cycle (C3 cycle).

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