The correct answer is A. Sheet3!A10.
To refer to a cell on another sheet, you need to precede the cell reference with the name of the sheet, followed by an exclamation point. In this case, the cell we want to refer to is A10 on sheet3, so the correct reference is Sheet3!A10.
Option B is incorrect because it does not include the exclamation point. Option C is incorrect because it does not include the name of the sheet. Option D is incorrect because it does not refer to a cell on another sheet.