Assertion : Principle of vaccination is based on the property of 'memory' of the immune system.
Reason : In vaccination a preparation of antigenic protein of
pathogens or inactivated/ weakened pathogen are introduced
into the body.

Options

A. If both Assertion & Reason as true & the Reason is a correct explanation of the assertion.
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the principle of vaccination is based on the property of 'memory' of the immune system.
- This means that once the immune system encounters a pathogen, it can remember it and respond more effectively upon subsequent exposures.

2. **Understanding the Reason**:
- The reason explains that vaccination involves introducing a preparation of antigenic proteins from pathogens or inactivated/weakened pathogens into the body.
- This introduction helps the immune system recognize the pathogen without causing the disease.

3. **Mechanism of Vaccination**:
- When the vaccine (which contains the antigens) is administered, the immune system responds by recognizing these antigens.
- The immune response includes the production of antibodies and the formation of memory cells (both T cells and B cells).

4. **Role of Memory Cells**:
- Memory T cells and B cells are stored in the body and remain there after the initial exposure to the vaccine.
- If the body encounters the actual pathogen in the future, these memory cells can quickly recognize it and mount a faster and stronger immune response.

5. **Conclusion**:
- Both the assertion and the reason are true. The assertion correctly identifies the principle of vaccination as being based on immune memory, and the reason accurately describes how vaccination works by introducing antigens to stimulate this memory.

### Final Answer:
- Both the assertion and the reason are true, and the reason correctly explains the assertion.