Explanation:

- Option 1: MU = TUn+2 – TUn+1

- This is incorrect and not the right formula for marginal utility (MU). MU should be the change in total utility when one extra unit is consumed, specifically (TUn - TUn-1).

- Option 2: MU = TU/Q

- This says marginal utility equals total utility divided by quantity, which is not correct. This is average utility, not marginal utility.

- Option 3: MU = TUn – TUn-1

- This is the correct formula for marginal utility. It measures the change in total utility as one more unit is consumed.

- Option 4: TU = ?MU

- This is correct. Total utility is the sum of marginal utilities of all units consumed.