What type of system does a process variable lack the ability to send information to the final control element?

An open loop system is characterized by its lack of feedback, meaning it does not have a mechanism to send information about the process variable back to the control element that influences its operation. In such systems, the controller issues commands to the final control element, but these commands are not adjusted based on the actual performance or output of the system. This can lead to inefficiencies or inaccuracies because the system does not respond to changes in the output or any disturbances that may occur.

In contrast, a feedback system, for example, utilizes information from the process variable to adjust the control input, ensuring that the system maintains desired performance. A cascade system involves multiple control loops where the output of one loop serves as the input to another, providing multiple levels of control and feedback. A feedforward system anticipates changes in conditions and adjusts inputs accordingly, but it is still more responsive than an open loop since it attempts to mitigate the effects of disturbances before they affect the process output.

Thus, an open loop system's defining feature is its lack of feedback to adjust or correct based on the process variable, solidifying it as the correct answer.