Answer:
d. them
Explanation:
Although the sentence does not specify much, we can infer that the order is given to the "servants", as it is very unlikely that a Lady (a title often used to refer to a woman to whom homage, respect or obedience is due) carry the task of bringing somebody else's horses, especially when there are servants who can do it, instead.
Thus, "them" is the most suitable answer as this object pronoun refers to the third person in the plural (They - the servants).
