The correct answer is B: The frequency at which the open-loop gain of the amplifier equals one. The gain-bandwidth product (or gain-bandwidth) of an operational amplifier is the frequency at which the open-loop gain equals one (unity gain frequency). This is a key specification that determines the op-amp's frequency response. The gain-bandwidth product is constant for a given op-amp. As frequency increases, the open-loop gain decreases. At the gain-bandwidth frequency, the gain has dropped to 1 (0 dB). This frequency determines the maximum useful operating frequency of the op-amp. For example, if an op-amp has a gain-bandwidth of 1 MHz, and you configure it for a closed-loop gain of 10, the bandwidth will be approximately 100 kHz (1 MHz / 10 = 100 kHz). This relationship is fundamental to op-amp circuit design.