Final answer:
The event illustrating Keesh's determination best is his successful return from a solitary hunt despite his youth and inexperience.
Step-by-step explanation:
In 'The Story of Keesh', the event that best illustrates the importance of Keesh's determination is b) Despite being very young and inexperienced, Keesh returns successfully from his solitary hunt. This action highlights Keesh's resolve and ability to persevere in the face of adversity and doubt from his tribe. His successful hunt demonstrates his determination to prove himself capable and to ensure the survival and well-being of both himself and his mother.
Option a, while showing fairness, does not directly reflect Keesh's determination in the same manner as his hunt. Option c shows his resolve but it is more focused on his intellect rather than his determination. Option d indicates his assertiveness in seeking better living conditions but not his determination in the face of a challenge.