The correct answer is (c) Ratio of two resistors. In a voltage series feedback amplifier, the closed-loop gain is primarily determined by the feedback network, specifically the ratio of two resistors. The feedback configuration uses resistive elements to establish the gain, and this ratio is inversely proportional to the overall gain of the amplifier. The gain formula for this configuration is: Gain = 1 + (Rf/Rin), where Rf is the feedback resistor and Rin is the input resistor. This means the gain depends on the external resistor ratio rather than the inherent open-loop gain of the op-amp, making the amplifier design independent of the open-loop characteristics. This is one of the key advantages of using feedback in amplifier circuits.