a
\(Moles = \frac{{MV\,ml}}{{1000}} = \frac{{{{10}^{ - 3}} \times 10}}{{1000}} = {10^{ - 5}}\,mole\)

\(10^{-5}\) \(N_A\) molecules covering area \(= 0.24\,cm^2\)

\(1\) molecules covering area \( = \frac{{0.24}}{{{{10}^{ - 5}}\,{N_A}}}\)

\(\frac{{0.24}}{{{{10}^{ - 5}}\, \times 6 \times {{10}^{23}}}} = {a^2}\)

\(a^2 = 4 \times 10^{-20}\,cm^2\)

\(a = 2 \times 10^{-10}\,cm\)

\(a = 2 \times 10^{-12}\,cm\)

\(a = 2\,pm\)