Of all the calls a modern fire department responds to, the riskiest is:

Structure fires are considered the riskiest calls for fire departments due to several critical factors. These incidents often involve hidden dangers such as structural collapse, which can occur rapidly and without warning. Firefighters often face reduced visibility due to smoke, high temperatures, and the potential for backdrafts or flashovers, which can exacerbate the already perilous situation.

Additionally, structure fires typically involve civilian occupants, increasing the urgency of the response and consequently amplifying the risks to firefighters as they work to rescue individuals while also battling the fire. The variety of building materials, potential for hazardous conditions like trapped occupants, and variability in the structural integrity make these incidents particularly dangerous. Thus, the combination of these factors cements structure fires as the most hazardous among the calls a modern fire department responds to.