Final answer:
The correct sentence is option a) After the path had been plowed, we were able to walk through the park. This sentence correctly uses the past perfect tense to indicate the completion of one past action before another.
Step-by-step explanation:
The student's question involves choosing the correct sentence structure for expressing a completed action in the past that allowed for a subsequent action. The correct answer is a) After the path had been plowed, we were able to walk through the park. The use of 'had been plowed' indicates that the plowing of the path was completed before the subsequent action, which is walking through the park. This use of past perfect tense ('had been plowed') is appropriate when describing an action that was completed before another past action occurred.