To solve the given question, we need to fill in the blank with the correct verb form to complete the sentence in the context of the description provided. Let's analyze the options one by one:
Based on the explanation above, the correct answer is "Had been." It correctly conveys that "she" started working and continued doing so for six months up to the point of the speaker's visit in the past.
Therefore, the completed and grammatically correct sentence is: "When I visited them, she had been working for six months."