The power is transmitted through a transmission line of impedance (r+jx) at equal voltage at both ends. The maximum power can be transferred under the steady-state condition if

The power transfer capability of a transmission line is affected by its impedance ($r+jx$) and the voltage difference at its terminals. For equal voltage magnitudes, maximum power transfer is achieved when the phase angle difference between the voltages is equal to the impedance angle. The question asks for a condition on the line parameters ($r$ and $x$) that facilitates maximum power transfer.