What causes modern primers in cartridges to ignite?

The correct choice explains that modern primers in cartridges ignite primarily due to the impact of being struck by a hammer. When the firing pin of a firearm hits the primer, a small explosive charge within the primer is activated. This charge then ignites the gunpowder within the cartridge, leading to the discharge of the bullet.

This method of ignition is critical for the functioning of modern ammunition, as it relies on a mechanical impact rather than relying on external factors like air or heat. While some other options may involve aspects of firearm operation or different ignition systems, they do not accurately describe the primary mechanism used in most modern cartridges today. The straightforward impact method is key to ensuring reliable ignition in various shooting conditions.