The key number 8 is the correct one:-
3..4..5..6..7..8..9
I started from key No. 5 to open the door No. 1 ,I multiplied (1x5)=5,so I moved 5 steps,
6..7..8..9..3 to reach the key No. 3 which opens the 2nd door, again I multiplied(2x3)=6,
and I moved 6 steps,4..5..6..7..8..9 so the key No. 9 will open the door No.3,(3x9)=27,
by moving 27 steps we will reach the key No. 8....