Assertion: Purine bases present in DNA are adenine and guanine.
Reason: The base thymine is present in RNA while base uracil is present in DNA.

Options

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

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "Purine bases present in DNA are adenine and guanine."
- **Explanation**: Purine bases are a type of nitrogenous base that includes adenine (A) and guanine (G). In DNA, these two bases are indeed present. Therefore, the assertion is **true**.

### Step 2: Analyze the Reason
The reason states that "The base thymine is present in RNA while base uracil is present in DNA."
- **Explanation**: This statement is incorrect. In reality, thymine (T) is found in DNA, while uracil (U) is found in RNA. Therefore, the reason is **false**.

### Step 3: Conclusion
Since the assertion is true and the reason is false, we conclude that:
- The assertion is correct: Purine bases in DNA are adenine and guanine.
- The reason is incorrect: Thymine is present in DNA, not RNA, and uracil is present in RNA, not DNA.

### Final Answer
The correct option is that the assertion is true and the reason is false.

---