The reaction that is responsible for the primary fixation of `CO_(2)` is catalysed by

Options

A. RuBP carboxylase
B. PEP carboxylase
C. RuBP carboxylase and PEP carboxylase
D. PGA synthase

Explanation

To solve the question regarding which enzyme catalyzes the primary fixation of CO2, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the enzyme responsible for the primary fixation of carbon dioxide (CO2) in plants.

2. **Identify the Enzymes Listed**: The options given are:
- A) RUBP carboxylase
- B) PEP carboxylase
- C) Both RUBP carboxylase and PEP carboxylase
- D) PGA synthase

3. **Differentiate Between C3 and C4 Pathways**:
- In C3 plants, the primary fixation of CO2 is carried out by **RUBP carboxylase** (also known as RuBisCO). This enzyme catalyzes the reaction between ribulose bisphosphate (RUBP) and CO2.
- In C4 plants, the primary fixation of CO2 is carried out by **PEP carboxylase**. This enzyme catalyzes the reaction between phosphoenolpyruvate (PEP) and CO2.

4. **Analyze the Options**:
- **Option A (RUBP carboxylase)**: Correct for C3 plants.
- **Option B (PEP carboxylase)**: Correct for C4 plants.
- **Option C (Both)**: This option acknowledges both pathways but does not specify which is the primary fixation enzyme.
- **Option D (PGA synthase)**: This enzyme is not involved in the fixation of CO2.

5. **Conclude**: Since the question asks for the primary fixation of CO2, the answer depends on the type of plant being referred to. However, if we consider the general context of photosynthesis in higher plants, the most commonly referenced enzyme for primary fixation is **RUBP carboxylase** in C3 plants.

### Final Answer:
The reaction that is responsible for the primary fixation of CO2 is catalyzed by **RUBP carboxylase**.

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