What caused the injury to Nick's dad?

The injury to Nick's dad was caused by an explosion from an RPG (rocket-propelled grenade). This type of injury typically results from military combat situations where such explosive devices are used. RPGs are known for their destructive power and can inflict serious harm or fatalities to individuals within their blast radius. Given the context of the question, it suggests a scenario involving military engagement or operations where an RPG might be encountered, making it a plausible cause of injury in that setting.

While other options like a car accident, a fall during training, or a gunshot wound are all possible causes of injury, they do not align with the specific nature and intensity typically associated with RPG injuries, which often occur in conflict zones and entail significant force and impact.