Answer:
C.) A personal letter written by a civil rights leader in 1966
I think the answer is B because their word choice made me feel sympathy for Mundra and what she went through for her to ache all over and even her heart.

The harmonic frequency refers to the resonant frequency that is an integral multiple of the fundamental frequency, harmonics are usually simple sinusoidal oscillation at a single frequency. where as Resonance frequency are frequencies that are not necessarily sinosudal or integral multiple of fundamental frequency and it may have complex oscillations made up of multiple harmonics ~
It's like if you play a piano with a single key, it will show harmonic frequency, but if you use multiple keys at a time it may show Resonance.