Verified Answer :

The main purpose of an
eyewitness account is letter B. To tell a story

An
eyewitness is a person that has 
actually seen an act or occurrence. Thus, he can better give a firsthand account of
the 
happening. He is then capable of telling a genuine version
of the story because he has personally seen everything that has happened. 

The rest of the choices are simply not the best answers.