Study the following statements regarding lactic acid bacteria `(LAB)` which are used to convert milk into curd.
(i) They produce acids that coagulate and partially digest the milk proteins.
(ii) A small amount of curd added to the fresh milk as an inoculum contains millions of `LAB`, which at suitable temperature, multiply and convert milk into curd.
(iii) Conversion of milk into curd improves its nutritional quality by increasing vitamin `B_(12)`.
(v) LAB may result in acidity in the stomach of human beings.
Which of the given statements are correct ?

Options

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

Explanation

To determine which statements regarding lactic acid bacteria (LAB) used in the conversion of milk into curd are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Understanding LAB**:
- Lactic acid bacteria, commonly known as LAB, are beneficial bacteria that play a crucial role in the fermentation process of milk into curd. They are primarily represented by the genus Lactobacillus.

2. **Analyzing Statement (i)**:
- **Statement**: "They produce acids that coagulate and partially digest the milk proteins."
- **Explanation**: LAB produce lactic acid during fermentation, which causes the milk proteins (casein) to coagulate, leading to the formation of curd. This statement is **correct**.

3. **Analyzing Statement (ii)**:
- **Statement**: "A small amount of curd added to the fresh milk as an inoculum contains millions of LAB, which at suitable temperature, multiply and convert milk into curd."
- **Explanation**: When a small amount of curd (which contains LAB) is added to fresh milk, the LAB multiply rapidly at an appropriate temperature, effectively fermenting the milk into curd. This statement is also **correct**.

4. **Analyzing Statement (iii)**:
- **Statement**: "Conversion of milk into curd improves its nutritional quality by increasing vitamin B12."
- **Explanation**: The fermentation process does enhance the nutritional profile of milk, and curd does contain higher levels of certain vitamins, including B12. Therefore, this statement is **correct**.

5. **Analyzing Statement (iv)**:
- **Statement**: "LAB may result in acidity in the stomach of human beings."
- **Explanation**: While LAB produce lactic acid, which can contribute to the acidity of the food, they do not cause harmful acidity in the stomach. Instead, they are beneficial for digestion. Thus, this statement is **incorrect**.

### Conclusion:
- The correct statements are (i), (ii), and (iii).
- The incorrect statement is (iv).

### Final Answer:
The correct statements regarding lactic acid bacteria (LAB) used to convert milk into curd are:
- (i) They produce acids that coagulate and partially digest the milk proteins.
- (ii) A small amount of curd added to the fresh milk as an inoculum contains millions of LAB, which at suitable temperature, multiply and convert milk into curd.
- (iii) Conversion of milk into curd improves its nutritional quality by increasing vitamin B12.