c
(c) As \(I\) is independent of \({R_6},\) no current flows through \({R_6}\) this requires that the junction of \({R_1}\) and \({R_2}\) is at the same potential as the junction of \({R_3}\) and \({R_4}\). This must satisfy the condition \(\frac{{{R_1}}}{{{R_2}}} = \frac{{{R_3}}}{{{R_4}}},\) as in the Wheatstone bridge.