The correct answer is D) precession. When a force is applied to a gyrocompass, the gyroscope responds by undergoing a phenomenon called precession. Precession is the change in the orientation of the gyroscope's axis of rotation in response to an applied force. This is a key characteristic of gyroscopic devices and is the basis for how gyrocompasses work. The other answer choices are incorrect because: A) Gravity effect refers to the influence of gravity on the gyroscope, not its reaction to an applied force. B) Earth rate refers to the rotation of the Earth, which causes the gyrocompass to align with true north, not its reaction to an applied force. C) Gyroscopic inertia is the property of a gyroscope to resist changes in its axis of rotation, not its specific reaction to an applied force.