The correct answer is C) 4 milliamps to 20 milliamps. This is the industry standard for transmission of analog signals for a measured variable in a central operating system configured for supervisory control. The 4-20 mA current loop is a widely adopted standard in process control and industrial automation applications, as it provides a reliable and noise-resistant method of transmitting analog signals over long distances. The other options are incorrect because they do not represent the established industry standard. Option A (1 volt to 5 volts) and Option B (-10 volts to +10 volts) are less common voltage-based standards, while Option D (10 milliamps to 50 milliamps) is not a recognized standard for analog signal transmission.