Assertion (A) : Adenine cannot pair with cytosine.
Reason ® `:` Because there wold be two hydrogen atoms in one of the bonding and none at the other.

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 "Adenine cannot pair with cytosine." This is a claim about the base pairing rules in DNA.

2. **Understanding the Reason**: The reason provided is "Because there would be two hydrogen atoms in one of the bonding and none at the other." This suggests a misunderstanding of how base pairing works.

3. **Identifying Base Pairing Rules**: In DNA, there are two types of nitrogenous bases: purines and pyrimidines. Adenine (A) and guanine (G) are purines, while cytosine (C) and thymine (T) are pyrimidines. The base pairing rules state that:
- Adenine pairs with Thymine (A-T) via two hydrogen bonds.
- Guanine pairs with Cytosine (G-C) via three hydrogen bonds.

4. **Analyzing the Assertion**: The assertion is true because adenine does not pair with cytosine. Instead, adenine pairs with thymine.

5. **Analyzing the Reason**: The reason provided is misleading. The number of hydrogen bonds is not the only factor that determines base pairing. The specific structure and size of the bases also play a crucial role. Adenine and cytosine do not complement each other structurally, which is why they cannot pair.

6. **Conclusion**: The assertion is true, but the reason is not a correct explanation for the assertion. Therefore, the correct answer is that the assertion is true, but the reason is false.

### Final Answer:
- Assertion (A) is true: Adenine cannot pair with cytosine.
- Reason (R) is false: The explanation regarding hydrogen bonds is incorrect.