Final answer:
The pastor shortened his sermon to make time for an unplanned prayer vigil. The correct form is 'shortened' indicating a past action completed by the subject, the pastor.
Step-by-step explanation:
The correct answer to the sentence completion question is: The pastor shortened his sermon to allow time for an unplanned prayer vigil.
Here, the past tense form of the verb 'to shorten' should be used to indicate that the action has already taken place. Given that the context implies a past action, 'shortened' (Option A) is the correct choice. The other options, 'shortens' and 'shorten,' represent present tense forms inappropriate for this past event sentence structure.
Therefore, when completing the sentence with appropriate subject-verb agreement, it becomes: The pastor shortened his sermon to allow time for an unplanned prayer vigil.