Select the correct statements regarding the characteristics of acquired immunity.
(i) Cell-mediated immunity is responsible for acquired immunity.
(ii) It produces a primary response of low intensity.
(iii) Active and passive immunity are types of acquired immunity,
(iv) Polymorphonuclear leucocytes and natural killer cells are involved in acquired immunity.

Options

A. (i), (ii) and (iii)
B. (i), (iii) and (iv)
C. (i) and (iv)
D. (i) and (iii)

Explanation

To solve the question regarding the characteristics of acquired immunity, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Understanding Acquired Immunity**:
- Acquired immunity is the immunity that develops throughout an individual's life, as opposed to innate immunity, which is present at birth. Acquired immunity can be further divided into two main types: cell-mediated immunity and humoral immunity.

2. **Evaluating Statement (i)**:
- "Cell-mediated immunity is responsible for acquired immunity."
- This statement is **correct**. Cell-mediated immunity is indeed a component of acquired immunity and involves T-cells that help in recognizing and destroying infected cells.

3. **Evaluating Statement (ii)**:
- "It produces a primary response of low intensity."
- This statement is also **correct**. The primary immune response, which occurs when the body first encounters a pathogen, is typically of lower intensity compared to the secondary immune response.

4. **Evaluating Statement (iii)**:
- "Active and passive immunity are types of acquired immunity."
- This statement is **correct**. Acquired immunity can be classified into active immunity (where the body produces its own antibodies) and passive immunity (where antibodies are received from an external source).

5. **Evaluating Statement (iv)**:
- "Polymorphonuclear leucocytes and natural killer cells are involved in acquired immunity."
- This statement is **incorrect**. Polymorphonuclear leucocytes (such as neutrophils) and natural killer (NK) cells are part of innate immunity, not acquired immunity. They play a role in the immediate defense against pathogens.

### Conclusion:
Based on the evaluation of the statements:
- **Correct Statements**: (i), (ii), and (iii)
- **Incorrect Statement**: (iv)

Thus, the correct answer is that statements (i), (ii), and (iii) are true regarding the characteristics of acquired immunity.