b
When bulb glows with full intensity, then voltage across it will be $1.5\, V$ and voltage across $3$ $\Omega$ resistance will be $4.5\, V$.
Current through $3$ $\Omega$ resistance $i = \frac{{4.5}}{3} = 1.5\,A$
Same current will flow between $X$ and $Y$
So ${V_{XY}} = i{R_{XY}}$ $\Rightarrow $ $1.5 = 1.5\,{R_{XY}}$ $\Rightarrow $ ${R_{XY}} = 1\,\Omega $
