Final answer:
Viktor Frankl, author of 'Man’s Search for Meaning,' believed that finding meaning or purpose (d) is essential for a fulfilling life, a concept developed through his harrowing experiences in a World War II concentration camp.
Step-by-step explanation:
Viktor Frankl concluded that d) Finding meaning or purpose is the key to living a life that is fulfilling despite one’s circumstances. In his book “Man’s Search for Meaning,” written after surviving a German concentration camp during World War II, he argues that life's meaning comes from enduring suffering and choosing how to cope with it, find meaning in it, and move forward with renewed purpose. Frankl's work contrasts with the perspectives of other existentialists like Nietzsche who emphasized the pursuit of power, or utilitarians who seek the greatest happiness for the greatest number, showing that the search for individual meaning is distinct from power or happiness.