Check pronoun agreement in both the underlined and non-underlined sections carefully. In the original underlined segment, there's nothing for "they" to refer to, except "fisherman," a singular noun. "Their" in the non-underlined part also tells you that the subject needs to be plural. The need for "fishermen" narrows down the possibilities to (C) or (E).
Now the choice is between "have done" and "had." You use "had," in (E), to indicate action that occurs earlier in the past than another past action: "He had completed the assignment before he fell asleep." Notice the word "still" appears in each choice. You need to indicate that the action is continuing, not that it began and ended way in the past. (C)'s verb, "have done," reflects continuing action.
