Final answer:
The top moving part that travels from left to right when keys are struck on a typewriter is the carriage. It moves to allow space for each subsequent character to be printed after a key is pressed.
Step-by-step explanation:
The top moving part that travels from left to right when keys are struck or when it is pressed is the carriage. This component is a staple part of a typewriter. When keys on a typewriter's keyboard are pressed, the typebars with the corresponding letters hit the ribbon to make an imprint on the paper, and the carriage moves to allow the next character to be printed. The space bar is used to add spaces between words and doesn't contribute to the lateral movement of the carriage.