Final answer:
The correct answer is 'had already finished,' which is an example of the past perfect tense, indicating that the lecture was completed before they arrived.
Step-by-step explanation:
The correct answer to the question "The lecture ........ by the time they got there." is a) had already finished. This is because the sentence is describing an action that was completed before another action in the past. In English grammar, this is called the past perfect tense. It is used to talk about an event that was completed before another event or point in time in the past. For example, 'By the time they arrived at the cinema, the film had already started.'