In a plane electromagnetic wave travelling in free space, the electric field component oscillates…

Frequency of oscillation $=2\times {10}^{10} \mathrm{Hz}$
Given $c=3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$,
Electric field amplitude $E_0=48 \mathrm{V} {\mathrm{m}}^{-1}$
$\text{Using the formula, }c=\frac{E_0}{B_0}$
The amplitude of the oscillating magnetic field
$B{}_0=\frac{E{}_0}{c}=\frac{48}{3\times 10^8}=1.6\times 10^{-7} \mathrm{T}$.