Final answer:
A decrease in blood pressure results in increased sympathetic stimulation, leading to vasoconstriction and an increased cardiac output, which helps to normalize the blood pressure.
Step-by-step explanation:
When there is a decrease in blood pressure, the body responds through a mechanism known as the baroreceptor reflex. A decrease in blood pressure would cause a decrease in the rate of baroreceptor firing. This, in turn, triggers an increase in sympathetic stimulation of the heart and peripheral vessels. The sympathetic nervous system response causes an increase in cardiac output by making the heart beat faster and stronger. Additionally, it causes vasoconstriction of blood vessels, which increases vascular resistance and helps to raise blood pressure. Therefore, the correct response to a decrease in blood pressure is A) Sympathetic stimulation and vasoconstriction.