a) The opportunity cost of treating the leukemia patient is that you cannot treat other patients who would have high chance of surviving.
b) The opportunity cost of not treating him would be you could treat other patients who have high chance of survive.
c) It can be justified in an economic point of view. The opportunity cost is way more beneficial than treating the patient.
d) This is a typical case in a sense that you have to make choices that has best utility.