INS systems primarily measure which of the following forces?

Inertial Navigation Systems (INS) primarily measure acceleration to determine an aircraft's position and orientation. These systems utilize accelerometers to detect changes in velocity over time, which allows them to track the movement of the aircraft through space. By integrating the acceleration measurements over time, the INS calculates the current velocity and position relative to a starting point, enabling accurate navigation without relying on external references.

While gravity, wind, and magnetic forces can influence flight, they are not the primary measurements that INS systems rely on. Gravity is often accounted for in the system's algorithms but is not directly measured as a primary input. Wind effects can be navigated using other systems like Air Data or GPS, which provide context about the aircraft's performance in atmospheric conditions but are not part of the INS's core function. Magnetic fields can be utilized in other navigational systems, such as magnetometers, but they do not play a role in the acceleration measurements essential to an INS. Therefore, understanding that INS focuses on acceleration to derive navigational data clarifies why this choice is the correct answer.