Explanation: The voltage gain of a triode audio amplifier is determined by several factors, each playing a crucial role. A) **Tube transconductance (gm or mutual conductance):** This is an inherent characteristic of the triode, representing the change in plate current for a given change in grid voltage (ΔIp/ΔVg). A higher transconductance means the tube is more sensitive and can produce a larger change in plate current for the same input signal, directly contributing to higher gain. B) **Plate load impedance (RL):** The output signal voltage is developed across the plate load impedance as the varying plate current flows through it. According to Ohm's law, a larger load impedance will produce a larger voltage drop for a given change in plate current, thereby increasing the voltage gain. C) **Transformer step-up (if used):** If an output transformer is employed, its turns ratio can provide additional voltage step-up. A transformer with a secondary winding having more turns than its primary will amplify the voltage, further increasing the overall gain of the stage. Since all three elements directly influence the amplifier's gain, "All of the above" is the correct choice.