The correct answer is: a) A certain number of passengers are on board.
Shared taxis in Sikkim operate on a fixed route and depart when a certain number of passengers are on board. This is to ensure that the taxi is full and that the driver can make a profit. The number of passengers required to depart varies depending on the route and the time of day.
Option b) is incorrect because shared taxis in Sikkim do not depart at a scheduled time. They depart when a certain number of passengers are on board.
Option c) is incorrect because the driver does not decide when the shared taxi departs. The shared taxi departs when a certain number of passengers are on board.
Option d) is incorrect because tickets are not required to board a shared taxi in Sikkim. Shared taxis are a form of public transportation and are available to anyone who wants to use them.