[A]: In `C_4` plants, `CO_2` fixation occurs at three sites.
[R]: `CO_2` is fixed in mesophyll cells, bundle sheeth cells and vascular cambium

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding C4 Plants**: C4 plants have a unique mechanism for carbon fixation that allows them to efficiently utilize CO2, especially in conditions of high light intensity and temperature.

2. **Sites of CO2 Fixation**: In C4 plants, CO2 fixation occurs primarily in two types of cells:
- **Mesophyll Cells**: These cells are located in the leaf and are the first site where atmospheric CO2 is captured.
- **Bundle Sheath Cells**: These cells surround the vascular tissue and are the second site where CO2 is further processed.

3. **The Role of Enzymes**: The enzyme involved in the initial fixation of CO2 in mesophyll cells is **Phosphoenolpyruvate (PEP)**. PEP combines with CO2 to form **oxaloacetic acid** (OAA).

4. **Transport to Bundle Sheath Cells**: The oxaloacetic acid is then transported to the bundle sheath cells, where it is converted into another compound that is processed further by the enzyme **RUBP carboxylase** (RuBisCO) to continue the photosynthetic process.

5. **Conclusion**: The statement in the question is incorrect because CO2 fixation in C4 plants occurs only in mesophyll and bundle sheath cells, not in the vascular cambium.

### Final Answer:
- **[A]** is correct that CO2 fixation occurs at three sites, but **[R]** is incorrect because CO2 is fixed only in mesophyll cells and bundle sheath cells, not in the vascular cambium.