​Four words have been given, out of which three are alike in some manner and one is different. Select the odd one. ​

Analyze the vehicles
Motorcycle: A two-wheeled vehicle that typically carries one or two passengers.
Van: A four-wheeled vehicle, generally larger, used for transporting goods or multiple passengers.
Car: A four-wheeled vehicle used for transporting passengers.
Bus: A large, four-wheeled vehicle designed to carry many passengers

Motorcycle is a two-wheeled vehicle, while the others (Van, Car, Bus) are all four-wheeled vehicles.

Thus, option (a) is correct.