Final answer:
Oskar's grandfather decided to stop writing to his son after 9/11 due to a personal incident related to the emotional impact of the terrorist attacks.
Step-by-step explanation:
Oskar's grandfather's decision to stop writing to his son after 9/11 was influenced by a personal incident. The trauma and aftermath of the September 11 terrorist attacks had a profound impact on individuals and society as a whole, shaping personal narratives and changing the course of people's lives and decisions. The attacks brought about a surge of emotions and reactions, from increased patriotism to fear and mistrust towards certain groups. In this context, Oskar's grandfather's choice appears to stem from the deeply emotional and personal response triggered by the events of 9/11, rather than being directly related to broader political events, economic concerns, or religious beliefs.