A condition that can occur in radial engines but is unlikely to occur in horizontally opposed engines is...

The correct answer is hydraulic lock, which can occur in radial engines due to their design and operational characteristics. In a radial engine, the arrangement of cylinders can lead to the accumulation of fluid—typically fuel or oil—within the cylinders when the engine is in certain positions. If the engine is started in this condition, the presence of liquid in a cylinder can prevent the piston from moving, leading to hydraulic lock.

Horizontally opposed engines, however, have a more linear arrangement of cylinders, which lessens the likelihood of liquid pooling in any single cylinder during the engine's resting position. This design significantly reduces the risk of hydraulic lock, making it an uncommon issue in horizontally opposed types.

Other conditions mentioned, such as fuel starvation, vapor lock, and backfiring, can occur in both types of engines and are primarily related to fuel delivery and ignition systems, rather than the physical layout of the engine itself. Thus, hydraulic lock stands out as a condition more characteristic of radial engines due to their unique structural arrangement.