Wave division multiplexing (WDM) is most similar to:

Amplitude modulation varies signal strength to encode data on a single carrier and does not involve splitting a medium into multiple simultaneous channels.

Frequency modulation varies the frequency of a single carrier wave to encode information and is a modulation technique, not a multiplexing scheme.

Time division multiplexing interleaves signals by assigning each a specific time slot, rather than separating them by frequency or wavelength.

Frequency Division Multiplexing (FDM) divides a transmission medium into multiple non-overlapping frequency bands, each carrying an independent signal simultaneously. WDM applies this same principle to optical fiber by assigning each data stream a distinct wavelength (optical frequency), allowing multiple channels to coexist on one fiber without interfering - making WDM essentially FDM implemented in the optical domain.

CSMA/CD is a media access control method for shared Ethernet networks and has no relationship to multiplexing multiple channels onto a single medium by frequency or wavelength.