[A]: Vestigeal organs are those parts of the body which are greatly reduced and are use- less.
[R]: These are the ruminants of once fully de- veloped organs which are gradually lost as these were no longer necessary

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 "vestigial organs are those parts of the body which are greatly reduced and are useless." This means that vestigial organs are remnants of organs that were once functional but have lost their primary function over time.

2. **Understanding the Reason**: The reason provided is that "these are the remnants of once fully developed organs which are gradually lost as these were no longer necessary." This implies that vestigial organs were once useful in our ancestors but have become redundant due to evolutionary changes.

3. **Analyzing the Assertion**: The assertion is true because vestigial organs, such as the appendix or the plica semilunaris, are indeed reduced in size and do not serve any significant function in the current organism.

4. **Analyzing the Reason**: The reason is also true. It accurately describes the nature of vestigial organs as remnants of fully developed organs that have lost their function over time due to changes in lifestyle or evolutionary adaptations.

5. **Establishing the Relationship**: Since both the assertion and the reason are true, and the reason correctly explains why the assertion is true, we conclude that the reason provides a valid explanation for the assertion.

6. **Final Conclusion**: Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.