Explanation: A directional coupler with an "attenuation" of -30 dB means the power sampled at the coupled port is 30 dB *below* the power on the main line. In radio theory, every -10 dB represents a power ratio reduction of 1/10. Therefore: * -10 dB is a 1/10 power ratio. * -20 dB is a 1/100 power ratio (1/10 * 1/10). * -30 dB is a 1/1000 power ratio (1/10 * 1/10 * 1/10). Since the coupled power measured is 100 milliwatts and this represents 1/1000th of the line power, the actual power on the line must be 1000 times greater. Calculation: Line Power = Measured Coupled Power * 1000 Line Power = 100 mW * 1000 = 100,000 mW Converting milliwatts to watts: 100,000 mW = 100 Watts. Therefore, option B is correct. Options A, C, and D would correspond to different dB attenuation (coupling) values (-20 dB, -40 dB, and -50 dB respectively), not -30 dB.