Which form of violence is primarily used by terrorists to instill fear in a population?

The correct choice is the calculated use of unlawful violence. This form of violence is a hallmark tactic of terrorism, as it seeks to create a climate of fear among civilians and compel attention towards the underlying political or ideological grievances that terrorists aim to address.

Terrorists strategically employ acts such as bombings, assassinations, and hostage-taking, all carefully planned to maximize impact and visibility. The intent is not just to cause disorder, but to send a message, induce panic, and disrupt the normal social fabric, thereby coercing governments or societies into action or concession.

In contrast, other options such as protests and legitimate political discourse highlight methods of expressing disagreement or dissent without resorting to violence. Defensive war actions, however, typically involve military response to aggression rather than premeditated acts of terror aimed at civilians. This distinction underscores why the calculated use of unlawful violence is specifically associated with terrorism and its aim to instill fear.