A message signal of frequency 100MHz and peak voltage 100V is used...

Modulating index is given by,
$\mu = \dfrac{{A}_{m}}{{A}_{e}}$
$= \dfrac{100V}{400V}$
$= 0.25$
The difference between the maximum frequency of modulated wave and the minimum frequency of modulated wave is given by
\Deltaf = {f}_\max - {f}_\min
=\left({f}_{c} + {f}_{m}\right) - \left({f}_{c} - {f}_{m}\right)$<br/>$= 2{f}_{m}$<br/>$= 2 ( 100MHz )$<br/>$= 2 \times {10}^{8}Hz