At which of the following frequencies does maximum permissible exposure have the lowest value?

Maximum Permissible Exposure (MPE) limits are designed to protect humans from the heating effects of RF radiation. These limits are strictest (meaning they have the lowest allowable value) at frequencies where the human body absorbs RF energy most efficiently. The human body acts very much like a receiving antenna, and it is highly resonant at Very High Frequencies (VHF), particularly between 30 MHz and 300 MHz. Because our bodies absorb the most RF energy in this specific frequency range, the safety limits must be set at their absolute lowest to prevent dangerous tissue heating. Among the choices provided, 50 MHz is the only frequency that falls squarely into this highly resonant VHF band. The 3.5 MHz frequency is too low, and frequencies like 440 MHz and 1296 MHz are too high to cause maximum whole-body resonance, meaning their allowable exposure limits are actually higher.