$ The term chromosome was introduced by Waldeyer.
! Sex chromosomes are also called Heterosomes.

Options

A. If both As and R are correct and R is correct explanation of As
B. If both As and R are correct but R is not correct explanation of As
C. If As is correct, R is wrong
D. If both As and R are wrong.

Answer

### Step-by-Step Text Solution:

1. **Understanding the Assertion**:
- The assertion states that the term "chromosome" was introduced by Waldeyer.
- This is correct. The term "chromosome" comes from the Greek words "chroma" meaning color and "soma" meaning body, and it was indeed introduced by Waldeyer in the late 19th century.

2. **Understanding the Reason**:
- The reason provided is that sex chromosomes are also called heterosomes.
- This is also correct. Sex chromosomes (X and Y in humans) are referred to as heterosomes because they differ in shape and size, unlike autosomes which are homologous pairs.

3. **Analyzing the Relationship**:
- The assertion is correct, and the reason is also correct. However, we need to determine if the reason is the correct explanation of the assertion.
- The reason explains the classification of sex chromosomes but does not explain why the term "chromosome" was introduced by Waldeyer.

4. **Conclusion**:
- Since both the assertion and reason are correct, but the reason does not correctly explain the assertion, we conclude that the correct option is that the assertion is correct and the reason is correct, but the reason is not the correct explanation of the assertion.

5. **Final Answer**:
- The correct answer is option B: Assertion is correct, Reason is correct, but Reason is not the correct explanation of Assertion.