(d) Current through arm $ABC$,
$= 4/(40 + 60) = 0.04\ A$
Potential difference across $A$ and $B$,
$V_{A} - V_{B} = 0.04 \times 40 = 1.6V$
Current through arm $ADC$,
$= 4/(90 + 110) = 4/200A$
Potential difference between $A$ and $D$,
$V_{A} - V_{D} = \frac{4}{200} \times 90 = 1.8V$
$\therefore V_{B} - V_{D} = \left( V_{A} - V_{D} \right) - (V_{A} - V_{B})$
$1.8 - 1.6 = 0.2\ V$