An amplitude-modulated wave is represented by, C _ m ( t ) = 10 ( 1...

The function of the amplitude-modulated wave is represented by, ${C}_{m} = {A}_{C}\left[1 + \dfrac{{A}_{m}}{{A}_{C}}\sin 2\pi{f}_{m}t\right]$, now compare it from the given equation,
${C}_{m} ( t ) = 10 ( 1 + 0 . 2\cos 12560t ) \sin \left(111 \times {10}^{4}t\right)V$

$⇒ 2\pi{f}_{m} = 12560{s}^{- 1}$$⇒ {f}_{m} = 2000Hz = 2kHz$.