RuBP is

Options

A. First stable compound PCR cycle
B. Substrate for reduction phase
C. Last compound of carboxylation phase
D. Primary `CO_2` acceptor of RPP cycle

Answer

### Step-by-Step Solution:

1. **Identify the Full Form of RuBP**:
- RuBP stands for Ribulose 1,5-bisphosphate. It is important to know the full name to understand its role in photosynthesis.

2. **Understand the Role of RuBP in Photosynthesis**:
- RuBP is a crucial molecule in the Calvin cycle (also known as the C3 cycle). It acts as the primary acceptor of carbon dioxide (CO2) during the process of photosynthesis.

3. **Recognize the Function of RuBP**:
- In the Calvin cycle, RuBP combines with CO2 in a reaction catalyzed by the enzyme RuBisCO (Ribulose 1,5-bisphosphate carboxylase/oxygenase). This reaction is the first step in the process of carbon fixation.

4. **Determine the Correct Answer**:
- Among the options provided, RuBP is best described as the primary CO2 acceptor in the Calvin cycle (or RPP cycle). This is because it has a high affinity for CO2, allowing it to effectively capture carbon during photosynthesis.

5. **Conclusion**:
- Therefore, the correct answer to the question "RuBP is" is that it is the primary CO2 acceptor in the RPP cycle.