The correct answer is C: The differential input voltage needed to bring the open loop output voltage to zero. Op-amp input offset voltage is the differential input voltage (difference between the two input terminals) that must be applied to bring the open-loop output voltage to zero when no signal is applied. In an ideal op-amp, with both inputs at the same voltage, the output should be zero. However, real op-amps have small manufacturing variations that cause a small voltage difference between the inputs even when the output is zero. This required input voltage difference is the offset voltage. It's typically in the millivolt range. Offset voltage is an important specification because it causes errors in DC-coupled applications. Many op-amps have offset null adjustment pins to minimize this error.