The correct answer is A: E = I × R. The formula used to calculate voltage in a circuit is E = I × R, which is Ohm's Law. This fundamental relationship states that voltage (E, in volts) equals current (I, in amperes) multiplied by resistance (R, in ohms). Ohm's Law is one of the most important principles in electronics and electrical engineering. It describes the relationship between voltage, current, and resistance in a circuit. When you know any two of these values, you can calculate the third. The formula E = I × R means that voltage increases when either current or resistance increases. This law applies to DC circuits and is essential for understanding how circuits work, calculating power consumption, and designing electronic equipment.