[A] : `C_3 & C_4` cycle can not occur in the same plant
[R] : `C_3` cycle is common in dicot & `C_4` in monocots

Options

A. (a) If both A and R are true and R is the correct explanation of A
B. (b) If both A and R are true but R is not the correct explanation of A
C. (c) If A is true and R is false
D. (d) If both A and R are false

Answer

To solve the assertion-reason question, we will analyze both the assertion (A) and the reason (R) step by step.

### Step 1: Analyze the Assertion
The assertion states that "C3 and C4 cycles cannot occur in the same plant."

- **Explanation**: This statement is false. Both C3 and C4 photosynthetic pathways can occur in the same plant species. For example, certain plants like maize and sugarcane can exhibit both C3 and C4 pathways depending on environmental conditions and developmental stages.

### Step 2: Analyze the Reason
The reason states that "C3 cycle is common in dicots and C4 in monocots."

- **Explanation**: This statement is also false. While it is true that C3 photosynthesis is prevalent in many plants, it is not exclusive to dicots; it occurs in both dicots and monocots. In fact, C3 plants account for about 95% of all plant species. The C4 cycle, on the other hand, is found in a smaller number of plant species and is not limited to monocots either.

### Step 3: Conclusion
Since both the assertion and the reason are false, we conclude that the correct choice is option D, which states that both the assertion and the reason are false.

### Final Answer:
Both the assertion and the reason are false.

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