The lesson is teaching a basic rook and king versus king checkmate method. It is not the fastest. It is just basic, designed for someone probably rated below 1000.
Yes, you are correct that there were some alternatives. Did you lose some points by picking the "wrong" square? Gosh.That's terrible.
More importantly, do you think that a novice could learn anything from the lesson?
The lesson itself [BLA010] has 2 quizzes. In one of them, the answer of Rb6 is fastest, but so are Rd6, Rh6, Rg6 and Ra5. Also, from the starting position, using the box method of King and rook mate takes no more than 20 moves. The optimal mate is in 13 moves with Ra5 or Rd3, but the lesson gives a long 32 move solution. Why is there not a lesson on the box method of king and rook?