\(\mathrm{B}_{0}=20 \mathrm{nT}=20 \times 10^{-9}\, \mathrm{T}\)

(\(\because\)  velocity of light in vacuum \(\mathrm{C}=3 \times 10^{8}\, \mathrm{\,ms}^{-1}\)) 

\( \overrightarrow{\mathrm{E}}_{0} =\overrightarrow{\mathrm{B}}_{0} \times \overrightarrow{\mathrm{C}} \)

\( | \overrightarrow{\mathrm{E}}_{0} =|\overrightarrow{\mathrm{B}}| \cdot|\overrightarrow{\mathrm{C}}|=20 \times 10^{-9} \times 3 \times 10^{8} \)

\(=6 \,\mathrm{V} / \mathrm{m} \)