An epic simile can be defined as

a.) a comparison that describes the epic hero
b.) an extended comparison that uses like or as
c.) a comparison that uses lofty words and imposing language
d.) an extended comparison that says that one entity is something else

The answer is: an extended comparison that uses like or as.

An epic simile or Homeric simile is a lengthy (several lines long) comparison between one familiar thing and another less well-known thing or occurrence. It usually makes use of the words “like” or “as” to establish the comparison. The object of the epic simile is to make the reader comprehend the unfamiliar object of the comparison by means of the very simile. They usually appear at moments of turmoil or violence.  

the correct answer is the first one.