Final answer:
The tense used for an event that has been completed before the present moment is the past perfect tense, which is denoted by the auxiliary 'had' followed by a past participle. It expresses actions completed before another action in the past.
Step-by-step explanation:
The tense that is used for an event that has been completed before the present moment in time is the past perfect tense. This tense is formulated by using had followed by the past participle of the verb to express actions that were finished at some point in the past before another past action occurred. For example, 'She had finished her homework before dinner.'
The past perfect tense is often used to emphasize that one event occurred before another in the past. It can also indicate conditions that were in place for events or actions that might have taken place. To clarify this with examples, you could say 'By the time they arrived, the concert had already started,' which highlights the sequence of actions, or 'If they had left earlier, they might have caught the bus,' which talks about a hypothetical condition and its possible outcome in the past.
Since simple past, perfect progressive, and past perfect progressive were also mentioned in the answer choices, it's important to differentiate them. The simple past is used for a completed action or event at a specific time in the past, without any relation to another event ('They ate dinner at six o'clock'). The perfect progressive (more commonly called present perfect progressive) refers to an action that began in the past and continues into the present, or was recently completed ('They have been eating dinner for ten minutes'). The past perfect progressive tense describes a past, ongoing action that was completed before some other past action ('They had been eating dinner when I arrived').
The correct option for the student's question is b. past perfect.