I believe that the problem here is that your code is printing the shortest of the routes. The question states that Bus should always be picked over CNG or walking, even if it is slower. Similarly you should pick CNG instead of walking whenever possible.
I also notice you have also written
a<b && a<c && a<d, however Halum can still make it exactly on time if one of the routes matches the time he has to get there. This should then use
<= and not
Hope this helps.