Explanation: In a bipolar junction transistor (BJT), beta ($\beta$) is the common-emitter current gain, defined as the ratio of collector current ($I_C$) to base current ($I_B$). This gain is frequency-dependent; it remains relatively constant at lower frequencies but begins to decrease as the operating frequency increases, primarily due to internal junction capacitances. The beta cutoff frequency ($f_{\beta}$) is the specific frequency at which the common-emitter current gain ($\beta$) drops to 0.707 (or 1/√2) of its maximum low-frequency value. The 0.707 factor signifies the -3dB point, where the amplifier's power output has fallen to half its maximum. Therefore, option B is correct because it accurately describes the decrease in the common-emitter current gain (often simply referred to as "emitter current gain" in this context) to the defined 0.707 level at the cutoff frequency. Options A describes an increase, which is incorrect. Option C is vague and misses the "of maximum" qualifier. Option D refers to gate current gain, which is relevant to Field-Effect Transistors (FETs), not bipolar transistors.