In Google Sheets, the Ctrl + D shortcut performs the "Fill down" command. This action takes the content from the top cell of a highlighted range and copies it down into all the other selected cells in that column. This is a primary method for autofilling values or formulas vertically down a series of cells, matching the action described in the question for a column.

A. Ctrl + C is the standard shortcut for "Copy," which only places the cell's content onto the clipboard.

B. Ctrl + R is also a correct autofill shortcut, but it performs the "Fill right" command across a row, not down a column. The question allows for either, but Ctrl + D is a valid answer.

C. Ctrl + V is the standard shortcut for "Paste," which inserts content from the clipboard into the selected cell(s).

1. Google Docs Editors Help. (n.d.). Keyboard shortcuts for Google Sheets. Google. Retrieved from https://support.google.com/docs/answer/181110. (See the "Common shortcuts" section

which lists Ctrl + D for "Fill down").

2. Google Workspace Learning Center. (n.d.). Tips for Sheets: Fill data automatically. Google. Retrieved from https://support.google.com/a/users/answer/9300023. (This page describes the fill handle

for which Ctrl + D is the keyboard equivalent).