The series of lectures ___ delivered.

This question primarily tests subject-verb agreement and the correct use of passive voice in English grammar. Identifying the true subject of the sentence is crucial for selecting the appropriate verb form.

• The subject is "The series of lectures." Although "lectures" is plural, the head noun of the subject phrase is "series," which is singular. Therefore, the verb must agree with the singular "series."
• The sentence also requires a passive voice construction because "the series of lectures" is receiving the action of being delivered, rather than performing it.
• "has been" correctly uses the singular auxiliary verb "has" (agreeing with "series") and forms the passive voice with "been delivered."
• "has" (A) is incorrect as it's active voice and incomplete.
• "were" (B) and "have" (D) are plural auxiliary verbs, which incorrectly agree with the singular subject "series."

Thus, "has been" ensures both subject-verb agreement and correct passive voice.