Final answer:
The increased mean arterial pressure leads to a higher rate of firing by the baroreceptors.
Step-by-step explanation:
An increased rate of firing of the baroreceptors occurs when there is an increase in mean arterial pressure. Baroreceptors are stretch-sensitive mechanoreceptors located primarily in the carotid sinuses and aortic arch that respond to changes in blood pressure. With higher blood pressure, these receptors detect the increased stretch and initiate action potentials at a higher rate, thereby informing the cardiovascular center in the medulla oblongata to initiate homeostatic mechanisms to restore blood pressure to a normal level.
Therefore, out of the provided options, number 5 'Increased mean arterial pressure' would be an instance when there is an increased rate of firing of the baroreceptors.