Which factor most increases the risk that a combination vehicle will roll over in a curve or sudden maneuver?

Explanation

A raised center of gravity is what makes a combination vehicle far more likely to roll over. Fully loaded rigs are about ten times more likely to roll over than empty ones because cargo piled up raises the load's center of gravity, and keeping cargo low and centered are the two things that most reduce that risk. This is standard driver-training content; 49 CFR 383.111(c)(3)(i) names "safely operating combination vehicles" as a required-knowledge area without itself specifying rollover mechanics. Spreading cargo evenly and driving empty both describe conditions that reduce rollover risk rather than increase it, the opposite of what the question asks. A sliding fifth wheel is an unrelated coupling component with no connection to rollover mechanics.

Source: 49 CFR 383.111(c)(3)(i)