The answer is B. Horse.
In the pair “King : Throne”, the king sits on the throne. Similarly, in the pair “Rider : Horse”, the rider rides the horse.
The other options do not have the same relationship.
A. Seat is a general term for a place to sit, while a throne is a specific type of seat that is used by a king or queen.
C. Saddle is a piece of equipment that is placed on a horse’s back, while a horse is an animal that is ridden
D. Chair is a piece of furniture that is used for sitting, while a rider is a person who rides a horse.