Assertion: Photosynthetically `C_(4)` plants are less efficient than `C_(3)` plants.
Reason: The operation of `C_(4)` pathway requires the involvement of only bundle-sheath cells.

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 the Assertion**: The assertion states that "photosynthetically C4 plants are less efficient than C3 plants." To evaluate this, we need to know the efficiency of both types of plants in photosynthesis.

2. **C3 vs. C4 Plants**: C3 plants use the Calvin cycle directly for photosynthesis, while C4 plants have an additional step that helps them concentrate CO2 in bundle-sheath cells. This adaptation allows C4 plants to perform photosynthesis more efficiently, especially in hot and dry environments.

3. **Efficiency of C4 Plants**: C4 plants are generally considered more efficient than C3 plants because they can produce more ATP from one molecule of glucose. For instance, C4 plants can produce approximately 30 ATP, whereas C3 plants produce only about 18 ATP from the same amount of glucose.

4. **Photorespiration**: C4 plants also avoid photorespiration, a process that occurs in C3 plants where oxygen competes with carbon dioxide, leading to a waste of energy and carbon. Since C4 plants minimize this process, they are more efficient in terms of energy and carbon fixation.

5. **Understanding the Reason**: The reason states that "the operation of the C4 pathway requires the involvement of only bundle-sheath cells." This is incorrect because the C4 pathway involves both mesophyll cells and bundle-sheath cells. Mesophyll cells initially capture CO2 and convert it into a four-carbon compound, which is then transported to bundle-sheath cells for further processing.

6. **Conclusion**: Since both the assertion and the reason are false, the correct conclusion is that C4 plants are more efficient than C3 plants, and the operation of the C4 pathway involves more than just bundle-sheath cells.

### Final Answer:
Both the assertion and the reason are false. C4 plants are more efficient than C3 plants, and the C4 pathway involves both mesophyll and bundle-sheath cells.

---