The correct answer is B) Digital. Digital operating systems require the conversion of continuous analog signals from sensing and transmitting devices into discrete bits of digital information that can be processed by the system's computer. This conversion is necessary for the computer to understand and respond to the inputs from the various components of the system. The other options are incorrect because: A) Pneumatic systems use compressed air to transmit and control signals, without the need for digital conversion. C) Electric/electronic systems may utilize both analog and digital signals, but digital conversion is still required for computer processing. D) Direct connected systems refer to physical wiring connections, not the signal processing requirements.