Why would an image of a hero change over time?

a. to allow authors to produce a hero who is unique
b. to reflect the time and culture in which the hero was created
c. to show something completely opposite of what came before
d. to appeal to what modern-day audiences like and expect

I believe that an image of a hero would change over time D. to appeal to what modern-day audiences like and expect.
Heroes in the literature of the past were quite different from what we are used to in the modern day. So, the authors need to adapt their heroes a bit in order to suit the needs of modern-day audience who wants to read about something a little bit more contemporary.