Explanation: Transistor-Transistor Logic (TTL) refers to a family of digital integrated circuits where both the input and output stages, and often internal stages, primarily utilize bipolar junction transistors. This architecture provides fast switching speeds and good noise immunity, making it a very popular logic family from the 1960s through the 1990s for building digital circuits, including those found in radio equipment, microprocessors, and control systems. * **A) Resistor-transistor logic (RTL)** was an earlier, less efficient logic family. * **C) Diode-transistor logic (DTL)** was a predecessor to TTL, using diodes at the input. * **D) Emitter-coupled logic (ECL)** is a different, high-speed logic family with a distinct operating principle.