Final answer:
All of the functions listed (forming and concentrating urine, regulating blood pH and ion concentrations, and performing endocrine functions) are primary functions of the kidneys in the urinary system, so the correct answer is 'D) All of the above are primary functions of the kidneys.'
Step-by-step explanation:
The question asks which of the following is NOT a primary function of the kidneys in the urinary system. We need to consider the various roles the kidneys play in maintaining homeostasis. The kidneys indeed form and concentrate urine, regulate blood pH and ion concentrations, and perform endocrine functions such as producing erythropoietin (EPO) for red blood cell production and converting calcidiol to calcitriol to activate vitamin D. Therefore, all of the options given (A, B, C) represent primary functions of the kidneys, making the correct answer 'D) All of the above are primary functions of the kidneys.' The kidneys are crucial in not only waste elimination but also in several regulatory processes that ensure the body's fluids are balanced, blood pressure is controlled, and essential hormones are produced and activated.