Which of the following describes an engine where the pistons move horizontally rather than vertically or diagonally?

The flat or boxer engine configuration is characterized by its design where the pistons move horizontally. In this type of engine, opposing cylinders are arranged in a horizontal manner, which allows for a low center of gravity and a more balanced engine. This configuration is often found in vehicles such as the Subaru and certain models from Porsche, where the engine's design contributes to stability and handling.

In contrast, the straight or inline engine has pistons arranged in a single vertical line; the "W" configuration consists of two V-configured engines combined, creating more vertical height; and the "Vee" configuration has pistons arranged in two banks that form a "V" shape, which generally results in a vertical motion for the pistons. Therefore, the flat or boxer engine is distinct in its horizontal movement, making it the correct description for this type of engine.