b
Let the separation between plants is \(d.\) Therefore,

\(E =\frac{ v }{ d }\)

Substitute the values.

\(E =\frac{ v }{ d }\)

\(20 \times 10^{6}=\frac{20}{ d }\)

\(d =10^{-6} m\)

The expression of capacitance is given by,

\(C =\frac{\varepsilon_{0} A \varepsilon_{ r }}{ d }\)

Substitute the values.

\(9 \times 10^{-9}=\frac{\left(8.85 \times 10^{-12}\right) A (2.4)}{10^{-6}}\)

\(A=\frac{\left(9 \times 10^{-15}\right)}{\left(8.85 \times 2.4 \times 10^{-12}\right)}\)

\(=4.2 \times 10^{-4} m ^{2}\)