the one that is true Clydesdale and thoroughbred horses is : C. they are the same species and have the same traits.
To put it simply, you two organims could be categorized as 'the same species' if they can copulate and create an offspring.
In human for example, Asian and African had a very diffrent traits, but since every member of them can copulate and create an offspring, Asian and African are still in the same species.
