(Not a teacher)
1. If "D" means to not put anything in the blank, I would call it the best answer, but technically "who" should be "whom,": Whom do you think I met last Thursday? The use of "whom" is declining in English. I don't like answer "B; the "who" is redundant.
2. You can use "travel" if you drop the article "the," as in "How long does train travel from London to Glasgow take?" Also acceptable is "How long does travel by train from London to Glasgow take?" And, informally at least, "How long does the train from London to Glasgow take?" would also be understood.
Student or Learner