c
$\mathrm{R}_{\mathrm{eq}}=200+\frac{300 \times 600}{300+600}+100=500\, \Omega$
$I=\frac{100}{500}=\frac{1}{5} \mathrm{\,amp}$
$\mathrm{I}_{600}=\frac{\frac{1}{600}}{\frac{1}{300}+\frac{1}{600}} \times \frac{1}{5}=\frac{1}{15} \mathrm{\,amp}$
Reading of voltmeter $=I_{600} R_{600}=\frac{1}{15} \times 600=40 \mathrm{\,V}$