If the organizers wants to ensure that no participant should travel on the street from D to T, while equal number of participants should travel through junctions A and C, then a feasible set of task time (in hours) at junctions A, B, C, and D respectively to achieve this goal is:
0,5,2,2
No traffic flows from D - T. Now apply each of the options. New time will be as follows.
As it is given that traffic flow at junction A is same as that at junction C.
∴ Number of routes involving A that can be used must be same as that involving C.
Further, only the routes with minimum time can be used.
That happened in only (a), as no of the routes that can be used, the number of routes involving A is two (S-A-T and S-B-A-T) and that involving C is also two (S-B-C-T and S-D-C-T)