a
Using the condition of a balanced wheat stone bridge,
$\Rightarrow \frac{ R }{3}=\frac{4}{6} \Rightarrow R =2\,\Omega$
So the effective resistance of the circuit is
$R _{\text {eq }}=\frac{6 \times 9}{6+9}=\frac{18}{5}\,\Omega$
$i=\frac{36}{R_{e q}}=10\,A$