Since you need to round 3.984 to one decimal place, then that will be to the nearest tenth which 9. But the # at the right of 9 is 8, so it will be added by 1... 9 + 1 = 10 which is a 2-digit #, the 1 should be added now to 3, and what remains in the tenth digit is 0.
3.984 = 4.0 (final answer)