The correct option to complete the sentence "We ought to stay away .... the robbers come back" is D. in case. Step-by-Step Explanation:

1. Understanding the Sentence Context:
2. The sentence suggests a precautionary action. The phrase "We ought to stay away" indicates that there is a reason to avoid a certain situation, which in this case is the potential return of robbers.
3. Analyzing the Options:
4. A. provided: This word typically introduces a condition, meaning "if" or "on the condition that." For example, "We will go to the park provided it doesn't rain." In this context, it does not fit because we are not discussing a conditional situation but rather a precaution.
5. B. should in case: This phrase is not grammatically correct. The correct form would be "just in case" or "in case." The phrase "should in case" does not convey a clear meaning and is not standard English.
6. C. although: This word is used to introduce a contrast or exception. For example, "Although it was raining, we went for a walk." In this context, it does not fit because we are not contrasting two ideas; we are expressing a precautionary measure.
7. D. in case: This phrase is used to indicate a precaution taken to prepare for a possible future event. It means "as a precaution against" or "if something happens." In this sentence, it correctly conveys the idea that we should stay away as a precaution in the event that the robbers return.
8. Why Option D is Correct:
9. The phrase "in case" effectively communicates that the action of staying away is a precautionary measure against the possibility of the robbers coming back. It implies that we are taking steps to ensure safety in the event of a potential threat.
10. Common Pitfalls:
11. Confusing "in case" with "if" or "provided" can lead to incorrect sentence structures. It's important to remember that "in case" is specifically about taking precautions, while "if" and "provided" are more about conditions.
12. Misusing phrases like "should in case" can lead to grammatical errors. Always ensure that phrases are used in their correct forms.

Revision Summary:

• The correct answer is D. in case because it indicates a precautionary measure.
• "Provided" introduces conditions, which does not fit the context.
• "Should in case" is grammatically incorrect.
• "Although" introduces contrast, which is not relevant here.

By understanding the meanings and uses of these phrases, you can better choose the correct option in similar questions.