Answer: D). Field Horses
Step-by-step explanation:
Horses in groups are called in many names. But racehorses specifically are called in certain names.
Here are different types of horse group and what they are called:
Wild horses - Wild horses together are usually called a herd of horses, but a flock, mob, troop or band of horses are generally used too.
Stud horses - A group of stud horses is mostly called a harras of horses
Colt/Stallion horses - A group of colts are typically called a rag of horses.
Race horses - A racehorse group is usually called either a string or field of horses, but the term remuda of horses is also widely used.
Hope that helped a lot.
:)