Final answer:
The Backspace key is the one used to remove characters to the left of the cursor in a document, effectively backing up over them.
Step-by-step explanation:
The key that is used to back up over characters and remove them from a document is a. Backspace. When you press the Backspace key, the character to the left of the cursor (or insertion point) is deleted from the document. This is in contrast to the Delete key, which removes the character to the right of the cursor. The other keys listed, Esc and Tab, perform different functions. Esc typically is used to cancel a current task or command, and Tab is used to insert a tab character or move the cursor to the next tab stop in a document.