Explanation: A DC keep-alive potential is applied to a TR (Transmit-Receive) tube to enhance its performance. This small DC voltage partially ionizes the gas inside the tube, creating a small number of free electrons and ions. This pre-ionization makes the tube significantly more sensitive and responsive. When a strong transmitter pulse arrives, the pre-ionized gas can quickly and completely ionize (break down), forming a short circuit that protects the sensitive receiver from damage. Without the keep-alive potential, the ionization process would be slower, risking receiver damage. Furthermore, the partial ionization also helps the tube recover faster after the transmit pulse, allowing it to quickly pass weak incoming echo signals to the receiver, thus making it sensitive to receiver pulses as well. Therefore, the keep-alive potential both makes the tube more sensitive (to react quickly to transmit pulses and pass receive signals) and achieves this by partially ionizing the gas. Options A and B are partially correct but incomplete. Option C is incorrect because full ionization would continuously short the tube, preventing reception.