Final answer:
The optimal temperature for transporting hearts is 4 °C. Hearts for transplant need to be used within four hours, although new technologies have extended this time to 12 hours.
Step-by-step explanation:
The optimal temperature for transporting hearts to the processing facility is 4 °C (option a).
Hearts for transplant have to be used within four hours of the donor's death (as noted in the provided information). However, it is important to note that some advancements in technology, like the Organ Care System, have extended the time hearts can remain healthy and usable, up to 12 hours.