c
\(W = {U_2} - {U_1}\,\,\, = \frac{{k{q_1}{q_2}}}{{{r_2}}} - \frac{{k{q_1}{q_2}}}{{{r_1}}}\,\,\,\, = k{q_1}{q_2}\left[ {\frac{1}{{{r_2}}} - \frac{1}{{{r_1}}}} \right]\)

\( = 9 \times {10^9} \times 5 \times 10 \times {10^{ - 12}}\left[ {\frac{1}{{0.5}} - \frac{1}{1}} \right]\,\,\, = 45 \times {10^{ - 2}} \times (2 - 1)\,\,\,\,\,\,\)

\(\therefore \,\,\,W = 45 \times {10^{ - 2}}J\,\,\)