What is true of Clydesdale and thoroughbred horses?

They are different species and have different traits.

They are the same species and have the exact same traits.

They are the same species and have different traits.

They are different species and have the same traits.

The answer is

They are the same species and have different traits.

They are both horses, so they are the same species (latin name of the species: Equus ferus and both Clydesdale and thoroughbred hourses are also domesticated horses, Equus ferus caballus)

however, they are different: the Clydesdale horses are more calm and more useful for drawing (carriages) and thoroughbred horses are more active.

So the best answer is:

They are the same species and have different traits.