a
(a) For no force on wire \(C\), force on wire \(C\) due to wire \(D\) = force on wire \(C\) due to wire \(B\)
\( \Rightarrow \frac{{{\mu _0}}}{{4\pi }} \times \frac{{2 \times 15 \times 5}}{x} \times l = \frac{{{\mu _0}}}{{4\pi }} \times \frac{{2 \times 5 \times 10}}{{\left( {15 - x} \right)}} \times l\)

\( \Rightarrow x = 9\,cm.\)