To answer this question, you need to understand the basic commands used in the vi editor.
Option A) s - This option is incorrect because the s command is used to substitute characters. It deletes the character under the cursor and enters insert mode.
Option B) S - This option is incorrect because the S command is used to delete the current line and enter insert mode at the beginning of the line.
Option C) a - This option is incorrect because the a command is used to append text after the cursor. It enters insert mode after the current character.
Option D) i - This option is correct because the i command is used to insert text to the left of the cursor. It enters insert mode before the current character.
Therefore, the correct answer is option D) i.