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

In radial engines, a unique feature is their design, which can lead to conditions that are less likely to manifest in horizontally opposed engines. One such condition is hydraulic lock.

Hydraulic lock occurs when fuel or oil enters a cylinder and prevents the piston from moving because the liquid cannot be compressed. In radial engines, the layout often leads to fuel pooling in certain cylinders, especially if the engine is tilted or has not been operated in a while. This pooling can result in hydraulic lock, as the engine may not be able to expel this liquid before attempting to start, causing mechanical failure if the piston tries to compress the liquid.

In contrast, horizontally opposed engines are designed differently, allowing for better drainage and less likelihood of pooling in the cylinders. This means hydraulic lock is not a typical concern in those engines. As a result, the occurrence of hydraulic lock is more characteristic of radial engines, making it the correct answer for this question.