Final answer:
The verb 'exerting' in the sentence 'She had been exerting herself' is in the Past Perfect continuous tense.
Step-by-step explanation:
The verb 'exerting' in the sentence 'She had been exerting herself' is in the Past Perfect continuous tense. The Past Perfect continuous tense is formed by using the auxiliary verbs 'had been' followed by the present participle form of the verb, in this case, 'exerting'. It is used to indicate an ongoing action that started in the past and continued up until another point in the past.