Which type of multiplexing technique is used in Wavelength Division Multiplexing $($WDM$)?$

$15$

a$.$

Code Division Multiple Access $($CDMA$)$

b$.$

Orthogonal Frequency Division Multiplexing $($OFDM$)$

c$.$

Frequency Division Multiplexing $($FDM$)$

d$.$

Time Division Multiplexing $($TDM$)$