The function of the amplitude-modulated wave is represented by, $C_{\mathrm{m}}=A_{\mathrm{C}}\left[1+\frac{A_{\mathrm{m}}}{A_{\mathrm{C}}}\sin 2\pi f_{\mathrm{m}}t\right]$, now compare it from the given equation, 
$C_m\left(t\right)=10(1+0.2\cos 12560t)\sin \left(111\times {10}^{4}t\right) \mathrm{V}$

$\Rightarrow 2\pi f_{\mathrm{m}}=12560 {\mathrm{s}}^{-1}$$\Rightarrow f_{\mathrm{m}}=2000 \mathrm{Hz}=2 \mathrm{kHz}$.