The best word to complete the sentence is **B. when**.

Here's a detailed explanation of why "when" is the correct choice, and why the other options are not appropriate:

### Sentence in context:
"They left me at the bus stop ________ they had met me the week before."

1. **A. which**: This word is used to introduce relative clauses but is not appropriate in this context. It would imply a further description of a noun, which doesn't fit because the sentence structure needs a word that indicates the time of the meeting, not a clarifying detail.

2. **B. when**: This is the correct choice because "when" is an adverb that refers to time. It indicates that the event of them leaving you at the bus stop is directly related to the time they had met you. In other words, it establishes a temporal relationship between the two events.

3. **C. that**: This word can introduce restrictive clauses but does not fit in terms of showing time. It would imply a definition or restrictive condition, which doesn’t suit the context here as we are trying to indicate a specific point in time.

4. **D. where**: This word is used to indicate a place. While "where" might seem relevant since the bus stop is a location, the sentence is concerned with the timing of when they met, not the place. Thus, it does not appropriately complete the sentence.

### Conclusion
Therefore, the best option is **B. when**, as it indicates the time relationship between the two events described in the sentence. The complete sentence would read: "They left me at the bus stop **when** they had met me the week before."