An EM wave is propagating at a frequency ‘f’ is an air-filled rectangular waveguide having the cutoff frequency ‘f₀’. Consider the phase velocity of the EM wave in the waveguide is V_p. The plot of \(\left( {\frac{c}{{{v_p}}}} \right)\) versus \(\left( {\frac{{{f_0}}}{f}} \right)\) will be:

(c is the velocity of the wave in air)