Which correctly explains why a silicon ring resonator responds strongly only to specific wavelengths?

Answer: C. At wavelengths where the optical path length of one round trip is an integer multiple of the wavelength, the circulating light interferes constructively

In a ring resonator, part of the light in an adjacent bus waveguide transfers into the ring and circulates many times. At wavelengths where the optical length of one round trip (effective index × circumference) is an integer multiple of the wavelength, the phase lines up on every pass and the light builds up (resonance), causing a sharp dip in the light passing through the bus waveguide at that wavelength only. This property is used for filters that extract specific wavelengths and for compact modulators that shift the resonant wavelength electrically. There are multiple discrete resonant wavelengths, and rings with radii from a few µm to several tens of µm are used.