Now, the total time for each of the five routes will be as follows.
RouteTotal TimeS−A−T14+aS−B−A−T9+(a+b)S−B−C−T7+(b+c)S−D−C−T10+(c+d)S−D−T13+d
As B - C is unusable; S - B - C - T is not possible. From the remaining options, if we apply all of them:
RouteOption aOption bOption cOption dS−A−T161415S−B−A−T161415S−D−C−T151415S−D−T151415
Both option (b) and (c) are true, as in both the cases total time is same for each of the four routes.