A stroke destroying a few cubic centimetres of medulla is usually fatal, while a stroke destroying far more cerebellum leaves the patient alive and clumsy. Why does location matter so much more than volume here?

C. Because the medulla's functions have no substitute and the cerebellum's do.

The question is not how much tissue was lost but whether the function has a substitute. Nothing else can drive breathing, so the medulla's destruction stops it in minutes; movement is generated by cortex and cord and merely refined by the cerebellum. The cerebellum holds more neurons than the rest of the brain combined, so neither count nor volume predicts the consequence.