What mode activates automatically when the received signal is below an acceptable minimum value?

The option indicating "Memory mode" is the correct choice, as it signifies a specific functionality in pulse radar systems. When the received signal drops below an acceptable minimum threshold, the radar often switches to memory mode to preserve and record critical data. This capability allows the system to maintain important information about previous signals, providing the opportunity for analysis and troubleshooting, even when current reception is inadequate.

In contrast, standby mode typically refers to a state where the system is powered but not actively processing information, which would not serve the purpose of dealing with low signal scenarios. Normal mode, on the other hand, is the operational state during which the radar actively processes signals and does not automatically engage under suboptimal conditions. Error mode generally indicates a malfunction or issue in the system, which is distinct from the need to manage low signal reception effectively. Therefore, memory mode stands out as the correct answer due to its designed response to situations where the signal quality is insufficient.