Shyam wants the slowest possible trip, so this time we hunt for the maximum total time rather than the minimum. Leg 1 is always 12 hours no matter the mode. On leg 2 the slowest mode is bus at 16 hours. On leg 3, bus and taxi are tied for slowest at 15 hours. On leg 4, bus and taxi are tied for slowest at 5 hours. Adding these up, the highest total any itinerary can reach is $12 + 16 + 15 + 5 = 48$ hours.
Four of the five itineraries, I, II, III and IV, all reach the same 48 hour maximum because leg 1 does not matter (all its modes tie at 12 hours) and each of these four picks a slowest-or-tied-for-slowest mode on legs 2, 3 and 4. Only V breaks the pattern by using taxi instead of bus on leg 2.
Let's summarize:
So the itineraries that maximise travel time are I, II, III and IV.