Which safety guideline for using bows is unique to the crossbow?

The guideline that is unique to the crossbow among the options provided is to never dry fire a crossbow. Dry firing refers to releasing the string of a bow or crossbow without an arrow nocked. This can lead to significant damage to the crossbow and can be dangerous to the user as it may cause the limbs to fracture or break. Unlike traditional bows, crossbows have a different construction that makes them more susceptible to damage from dry firing.

While the guidelines about using a crossbow for short-range shooting, following safety rules for both firearms and bows, and never using a cracked arrow are important, they are not unique to crossbows specifically. The rule regarding dry firing is particularly critical for crossbow safety, making it the best answer in this context.