Which of the following components is primarily responsible for the high gain characteristic of an operational amplifier?

The high gain of an operational amplifier mainly comes from its first stage, the differential amplifier that uses a pair of transistors (or MOSFETs). This differential pair amplifies the small voltage difference between the two input terminals many times, creating a large output voltage. Because the transistors are biased in a region of high transconductance, even tiny input differences produce large output swings. For example, if the differential pair amplifies a 1 mV input by 10 000, the op‑amp can produce a 10 V output, showing the huge internal gain. This internal amplification is what gives the op‑amp its characteristic high gain.