When TU is maximum, what happens to MU?

MU is zero

The correct answer is Option (3) → MU is zero

When Total Utility (TU) is at its maximum, Marginal Utility (MU) is zero. This is because MU represents the change in TU with each additional unit consumed. When TU reaches its peak, any additional unit consumed does not increase the total utility further, hence MU is zero at that point.