Final answer:
The Enter key on the keyboard is used to complete a "hard return", which signals the end of a paragraph in word processing.
Step-by-step explanation:
The key on the keyboard that, when pressed, completes a "hard return" is A) Enter. A hard return is used in word processing to signify the end of a paragraph, creating a new line and often some space between the lines of text. This is different from a soft return, which is often used to create a new line without starting a new paragraph. In contrast, the Tab key is used for indentation, the Spacebar is used to insert spaces between words, and the Shift key is used to capitalize letters or to access the secondary function of a key.