1.26 kJ
First, calculate the normal force. That's merely the mass of the desk multiplied by the gravitational acceleration.
80 kg * 9.8 m/s^2 = 784 kg*m/s^2 = 784 N
Now calculate the force needed to slide the desk by multiplying by the coefficient of kinetic friction. So
784 N * 0.4 = 313.6 N
Work is defined as force times distance, so
313.6 N * 4 m = 1254.4 Nm = 1254.4 J = 1.2544 kJ
Rounding to 3 significant figures gives 1.25 kJ
Of the available choices, 1.26 kJ is the closest. Looks like a mild difference in rounding. Perhaps because I rounded after all calculations were done and they may have rounded intermediate results.
