Which sensor would indicate that a hydraulic cylinder is stationary?

The correct choice for indicating that a hydraulic cylinder is stationary is a contact limit switch. This type of switch is designed to detect the physical position of the hydraulic cylinder, which can show whether the cylinder is in a fully extended or retracted position, ultimately indicating that it has stopped moving.

A pressure switch measures the pressure within the hydraulic system and does not provide information about the motion or position of the hydraulic cylinder. A tachometer is typically used to measure the rotational speed of motors or other rotating components and wouldn't be relevant for linear motion like that of a hydraulic cylinder. A linear transducer also measures movement but is typically used to provide varying outputs based on the position of the cylinder instead of indicating a stationary state specifically.

In summary, a contact limit switch is directly suited for detecting whether the hydraulic cylinder is at a halt, making it the most appropriate choice among the options provided.