What is the purpose of an eyewitness account?
a.) to persuade
b.) to tell a story
c.) to promote a view
d.) to give instructions

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.