Final answer:
The principal part used to form the verb 'learned' is the past participle.
Step-by-step explanation:
The principal part used to form the verb 'learned' in the sentence 'In the Middle Ages, students learned from priests and other church officials' is the past participle.
The past participle is the form of a verb that is used to create the perfect tenses and also the passive voice. In this case, the verb 'learned' is formed by adding 'ed' to the base form of the verb 'learn'. Other examples of verbs formed using the past participle include 'cooked', 'written', 'seen', and 'played'.
In the sentence 'In the Middle Ages, students learned from priests and other church officials', the principal part used to form the verb 'learned' is the past tense.
This means the correct answer to the question is d) past. The past tense is used to indicate that an action was completed at some point in the past, which fits the context of the sentence describing historical events in the Middle Ages.