The form of a verb should be used to distinguish between actions completed in the immediate past and actions completed before a specific time.

a. infinitive

b. irregular

c. simple past

d. past perfect

The answer is: d. past perfect.

The Past Perfect tense is used to describe an action that happened before another action or a specific time in the past. It is formed with the auxiliary verb had and the past participle form of the verb. For example, "When the woman arrived home, her husband had cooked dinner," which means that the husband cooked dinner before the woman arrived.

The rest of the alternatives are incorrect because the infinitive and irregular forms of a verb are not past tenses by themselves. Finally, the Simple Past is used to refer to finished or completed actions in the past.