Correct answer is (b) 4 hours
Explanation: After the heart has been removed from the body, it can be kept alive for 4 hours. It is placed in a cool box where the temperatures are monitored to keep the heart alive. A miniature heart lung setup is also used where the blood is made to circulate in the heart and it is kept alive. When this heart reaches the site of transplant, the doctors assess if the heart is still healthy and fit enough to be transplanted. After this, the transplantation takes place.