Explanation: A mixer circuit operates as a non-linear device, designed to combine two input frequencies (let's call them f1 and f2). Due to this non-linear interaction, new frequencies are generated at the output. The principal frequencies that appear are the two original input frequencies (f1 and f2), along with their sum (f1 + f2) and their difference (f1 - f2). These sum and difference frequencies are crucial for processes like frequency conversion in superheterodyne receivers. Options A, B, and D are incorrect. Option A describes harmonics, which are multiples of a single frequency, not the output of combining two distinct frequencies in a mixer. Option B incorrectly includes "square root" as an output frequency. Option D refers to voltage ratios or constants, not frequencies generated by a mixer.