b
$15\,\Omega $ and $ 10\, \Omega$ are in parallel current through
$10\, \Omega$ resistance $=\frac{\mathrm{V}}{\mathrm{R}}=\frac{10}{10}=1 \mathrm{\,A}$
$I=\frac{10}{2}=5 \,A$
$\mathrm{V}_{\mathrm{AB}}=\mathrm{IR}_{\mathrm{eq}}$
$=5 \times 10=50 \mathrm{\,V}$
