Assertion : The heterocyclic compounds in nucleic acid are the nitrogenous bases.
Reason : Adenine and guanine are substituted pyrimidines while uracil, cytosine and thymine are substituted purines.

Explanation

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "The heterocyclic compounds in nucleic acid are the nitrogenous bases."
- Heterocyclic compounds are cyclic compounds that contain at least two different elements in their ring structure. In nucleic acids, the nitrogenous bases (adenine, guanine, cytosine, thymine, and uracil) are indeed heterocyclic compounds because they contain nitrogen and carbon in their structures.

2. **Evaluating the Reason**:
- The reason states that "Adenine and guanine are substituted pyrimidines while uracil, cytosine, and thymine are substituted purines."
- To evaluate this, we need to identify the correct classifications of these nitrogenous bases:
- **Adenine and Guanine**: These are classified as purines, not pyrimidines. Purines have a two-ring structure, while pyrimidines have a single-ring structure.
- **Cytosine, Uracil, and Thymine**: These are classified as pyrimidines, not purines. They have a single-ring structure.

3. **Conclusion**:
- Since the assertion is correct (the nitrogenous bases are indeed heterocyclic compounds), but the reason is incorrect (adenine and guanine are purines, not pyrimidines, and cytosine, uracil, and thymine are pyrimidines, not purines), we conclude that:
- **Assertion**: True
- **Reason**: False

### Final Answer:
- The assertion is correct, but the reason is incorrect.

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