The correct answer is C. had left.
The sentence is in the past perfect tense, which is used to talk about an event that happened before another event in the past. In this case, the event that happened before is “we reached the bus stand,” and the event that happened after is “the bus left.” The past perfect tense is formed with the past participle of the verb (left) and the auxiliary verb had.
A. left is in the simple past tense, which is used to talk about an event that happened at a specific time in the past. In this case, the sentence would mean that the bus left at a specific time before we reached the bus stand. However, the sentence does not give any information about when the bus left, so the simple past tense is not the correct tense to use.
B. has left is in the present perfect tense, which is used to talk about an event that happened at an indefinite time in the past or that is still happening. In this case, the sentence would mean that the bus left at some point in the past and is still not here. However, the sentence implies that the bus is not here because it left before we reached the bus stand, so the present perfect tense is not the correct tense to use.
D. will have left is in the future perfect tense, which is used to talk about an event that will happen before another event in the future. In this case, the event that will happen before is “we reach the bus stand,” and the event that will happen after is “the bus leaves.” However, the sentence implies that the bus has already left, so the future perfect tense is not the correct tense to use.