Which of the following key-combinations is used to redo a change undone by a u...

The Ctrl-r key-combination is used to redo a change undone by a u command in vim. You can go back in time with the undo command. You can then go forward again with the redo command. If you make a new change after the undo command, the redo will not be possible anymore.

Answer options B and D are incorrect. The Ctrl-w, s and Ctrl-w, v key-combinations are used to view a file in two windows in a single vim screen. Ctrl-w, s splits the screen horizontally and Ctrl-w, v splits the screen vertically.

Answer option A is incorrect. The Ctrl-w key-combination with an arrow key is used to jump from one window to another. This will move you to the next window in whichever direction the arrow would normally move your cursor.