Final answer:
Danger close is the closest distance at which artillery and mortars can fire without endangering friendly troops, indicating proximity of fire support to allied forces.
Step-by-step explanation:
The term "danger close" refers to a condition where artillery and mortars are fired in close proximity to friendly forces. In this context, it specifically means option (b): The closest distance at which artillery and mortars can fire without endangering friendly troops. This condition acknowledges the risk to friendly forces due to the proximity of the firing but is considered necessary to provide fire support against threats near friendly units.