The work done against the gravitational force is calculated using the formula W = mgh, where m is the mass of the object, g is the acceleration due to gravity (approximately 9.8ms2 on Earth), and h is the height. Substituting the given values, we get W = 3.0 kg * 9.8m∕s2 * 240 m = 7.2 kJ. Therefore, the correct answer is '7.2 kJ'.