Explanation:

- Option: 1, "it has turned sour" - This segment is problematic due to tense inconsistency. If you're talking about something that happens as a direct consequence of the action ("are put in the milk"), a present perfect tense like "it has turned sour" doesn't fit well. Instead, it should be "it turns sour" to indicate a direct result.

- Option: 2, "drops of lemon" - Grammatically correct. Although "juice" is commonly used, "drops of lemon" is understood.

- Option: 3, "If a few" - This is correct. It's a suitable way to introduce a conditional clause.

- Option: 4, "are put in the milk" - This is also grammatically sound. It denotes a present action.

Correct Answer: Option: 1, it has turned sour