Which device contains a suspended mass free to swing along the input axis within a fluid-filled case?

The device that contains a suspended mass free to swing along the input axis within a fluid-filled case is an accelerometer. An accelerometer works by detecting changes in motion and acceleration by measuring the displacement of the suspended mass relative to its rest position. This suspended mass, often referred to as a proof mass, experiences a force due to acceleration, causing it to move within the fluid-filled housing, enabling the sensor to determine the amount and direction of acceleration.

While a gyroscope measures angular velocity and orientation using the principles of angular momentum, it does not involve a suspended mass that swings freely within a fluid. Similarly, an inclinometer measures tilt or angle relative to a reference plane, but it typically relies on fluid movement or other sensing elements rather than a suspended mass. An altimeter, used for measuring altitude, works through pressure changes in the atmosphere rather than by utilizing the movement of a mass within a fluid.

Thus, the accelerometer stands out for its design and function as described, making it the correct answer.