Final answer:
Essential hypertension is the type of hypertension most patients have, which is primarily due to genetic and lifestyle factors, and is not a result of a known disease process or a necessary physiological state.
Step-by-step explanation:
Essential hypertension, also known as primary hypertension, A. is the type of hypertension that most patients have. It is not the result of a known disease process, which means that option B does not apply. Rather, it's often caused by a combination of genetic factors and lifestyle choices such as diet and exercise. Essential hypertension is not a necessary physiological state, so option C is incorrect. Overall, only option A applies to the definition of essential hypertension.