The correct answer is (a) 3.
A factor of safety is a number that is used to determine how much stronger a structure needs to be than the forces it will be subjected to. In this case, the factor of safety is 3, which means that the vehicle is designed to withstand 3 times the weight of 12 passengers. This is done to account for unexpected loads, such as a sudden stop or a collision.
Option (b) is incorrect because it is the number of passengers that the vehicle is designed for, not the factor of safety. Option (c) is incorrect because it is the reciprocal of the factor of safety. Option (d) is incorrect because it is the number of passengers that are actually sitting in the vehicle, not the factor of safety.