It's important to note that Hamlet's tragic end is the result of multiple factors working in combination. However, among the options provided, the most significant reason for Hamlet's tragic end is:
D. Inability to kill Claudius when he had the chance
Hamlet's inability to act decisively and seize the opportunity to kill Claudius, the murderer of his father, is a central element in the play's unfolding tragedy. His procrastination, self-doubt, and the moral and ethical dilemmas he faces contribute significantly to the tragic events that follow. While other elements also play a role in the tragedy, Hamlet's hesitation to avenge his father's murder is a primary factor.