The correct option is B The emperor's horse could not be challenged by any petty ruler.
The Ashvamedha Yajna was a Vedic horse sacrifice ritual.
It was performed by rulers to prove their imperial sovereignty. In this ritual, a horse was left to wander for a year. Anyone could challenge the authority of the king by fighting with the warriors who travelled with the horse. If no one stopped the horse in that one year, the horse would be sacrificed and the king would be declared as the undisputed ruler of the entire territory. Samudragupta is said to have performed this ritual, and he even issued coins commemorating his unchallenged paramountcy over the empire.