Which type of ordnance includes devices designed for illumination or marking?

The correct choice is Pyrotechnics because this category encompasses devices specifically designed to produce light and sound effects, often used for illumination or marking purposes. Pyrotechnics can include flares and other signaling devices that serve to light up an area or indicate a specific location or event. These devices are often utilized in various situations such as military operations, rescue missions, and entertainment, playing a crucial role in visibility and signaling.

The other options do not fit the definition of devices intended for illumination or marking. Explosives primarily focus on the rapid release of energy, causing destruction or damage, while firearms are primarily designed for launching projectiles. Rockets, although they can be used in illumination contexts (like in some military applications), are not inherently categorized under illumination or marking devices in the same way that pyrotechnics are. Thus, pyrotechnics specifically cover the intended use described in the question.