The correct option to complete the sentence "He had just entered the office .... the telephone rang" is D. when. Step-by-Step Explanation:

1. Understanding the Context: The sentence describes two actions happening in sequence: one action (entering the office) occurs first, and then another action (the telephone ringing) happens immediately after.
2. Analyzing the Options:
3. A. as: This word suggests that two actions are happening simultaneously. If we say "He had just entered the office as the telephone rang," it implies that both actions are occurring at the same time, which is not the intended meaning here. The ringing of the telephone is a separate event that occurs right after he enters.
4. B. before: This word indicates that one action happens prior to another. If we say "He had just entered the office before the telephone rang," it suggests that there is a gap between the two actions, which is not the case here. The use of "just" implies immediacy, so "before" does not fit well.
5. C. since: This word is used to indicate a starting point in time and is often used in contexts where something has been happening continuously from a certain point. For example, "He has been working here since 2020." In this context, it does not fit because we are not discussing a continuous action but rather two distinct events.
6. D. when: This word is used to indicate that one action occurs at the same time as another or immediately after. Saying "He had just entered the office when the telephone rang" clearly indicates that the ringing of the telephone happened right after he entered, which is the intended meaning of the sentence.
7. Conclusion: The best choice is D. when because it accurately conveys the sequence of events without implying simultaneity or a gap in time.

Revision Summary:

• Correct Answer: D. when
• "When" indicates immediacy between two actions, fitting the context of the sentence.
• "As" suggests simultaneity, which is incorrect in this context.
• "Before" implies a gap between actions, which contradicts the use of "just."
• "Since" indicates a starting point for a continuous action, not applicable here.

By understanding the nuances of these conjunctions, you can better choose the correct option in similar sentence completion tasks.