The correct answer is B) Measuring the battery voltage with a digital voltmeter during charge or discharge. This is the most reliable and preferred method for determining the state of charge of a wet cell NiCad battery. The battery voltage provides a direct indication of the charge state, as the voltage will vary depending on whether the battery is being charged, discharged, or at rest. Measuring the specific gravity (options A and D) can also provide information, but is less direct and accurate than a voltage measurement. Additionally, a solenoid-type voltage tester (option C) is less precise than a digital voltmeter. For these reasons, measuring the battery voltage with a digital voltmeter is considered the most reliable method for assessing the NiCad battery's state of charge.