The correct answer is: C. Either I or II is implicit.
The statement “The X-Airlines has decided to increase the passenger fare by 15 percent with immediate effect” does not provide enough information to determine whether assumption I or assumption II is correct. It is possible that the demand for seats of X-Airlines may remain unchanged even after the hike of fare, or it is also possible that the demand for seats may decrease. Similarly, it is possible that other airline companies may also hike the passenger fares, or they may not.
Therefore, either assumption I or assumption II could be correct, but we cannot say for sure without more information.