Normal homeostatic mechanisms

Which of the following is an example of a situation in which events are severe enough that normal homeostatic mechanisms cannot fully compensate for them?
O a. Vasodilation leading to frostbite.
O b. Increase in heart rate in a transplant patient in response to running a marathon.
O C. Baroreceptor response to donating blood.
O d. Decrease in heart rate due to orthostatic hypotension