What characterizes a Horizontally Opposed Engine?

A horizontally opposed engine is defined by the arrangement of its cylinders, specifically having an even number of cylinders that are positioned directly opposite each other. This design allows for better balance and reduced vibrations within the engine, making it particularly advantageous for aircraft use. The opposing cylinders work in pairs, which contributes to a smoother operation and efficient performance. This engine type is commonly found in many general aviation aircraft, providing reliability and effective power delivery.

The other options involve engine designs that do not fit the definition of a horizontally opposed engine. The second option mentions an odd number of cylinders arranged in a circle, which pertains to rotary or radial engines. The third option describes a single cylinder engine, which is too simplistic and does not involve the distinct layout of a horizontally opposed configuration. The last option mentions a dual-engine configuration, which refers to the use of two engines rather than the specific arrangement of cylinders within a single engine.