a
\(\,F\,\, = \,\,\frac{{k{q^2}}}{{{r^2}}}\,\,\,\, \Rightarrow \,\,\,0.144\,\,N\,\, = \,\,\frac{{\left( {9\,\, \times \,\,{{10}^9}} \right)\,\,{q^2}}}{{{{\left( {5\,\, \times \,\,{{10}^{ - 2}}} \right)}^2}}}\)

\(q\,\, = \,\,0.2\,\, \times \,\,{10^{ - 6}}\,\,C\,\, = \,\,0.2\,\,\mu C\)