The correct answer is: C. was waiting
The sentence is in the past tense, so we need to use a past tense verb. The verb “wait” can be used in the simple past tense, the past progressive tense, or the present perfect progressive tense.
In the simple past tense, “wait” means to stay in one place for a period of time in order to meet someone or something. For example, “I waited for my friend at the bus stop.”
In the past progressive tense, “wait” means to be in the process of staying in one place for a period of time in order to meet someone or something. For example, “My friend was waiting for me at the bus stop when I arrived.”
In the present perfect progressive tense, “wait” means to have been staying in one place for a period of time in order to meet someone or something. For example, “My friend has been waiting for me for an hour.”
In this case, the best option is “was waiting” because it is the past progressive tense. The past progressive tense is used to describe an action that was in progress at a specific time in the past. In this case, the specific time in the past is when the speaker arrived.