Assertion : Only Lactobacillus lactic causes curdling of milk.
Reason: Only these bacteria can produce lactic acid.

Options

A. If both Assertion and Reason are true and Reason is a correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not a correct explanation of Assertion
C. If Assertion is true but Reason is false.
D. If both Assertion and Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Only Lactobacillus lactic causes curdling of milk." This implies that this specific bacterium is solely responsible for the curdling process in milk.

2. **Examining the Reason**: The reason provided is that "Only these bacteria can produce lactic acid." This suggests that only Lactobacillus lactic and possibly other bacteria mentioned can produce lactic acid, which is necessary for curdling.

3. **Evaluating the Assertion**:
- While Lactobacillus lactic does play a significant role in curdling milk by converting lactose (milk sugar) into lactic acid, it is not the only bacterium that can perform this function.
- Other bacteria, such as Lactobacillus acidophilus, Lactobacillus bulgaricus, and Streptococcus lactis, also contribute to the curdling process.

4. **Evaluating the Reason**:
- The reason states that only these bacteria can produce lactic acid. This is incorrect because multiple types of bacteria, not just Lactobacillus lactic, can produce lactic acid.

5. **Conclusion**: Since both the assertion and the reason are incorrect, we conclude that the correct answer is that both the assertion and reason are false.

### Final Answer:
Both the assertion and reason are false.

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