b
Current is constant in conductor

\(i=\) constant

Resistance of element \(d R=\frac{\rho d x}{\pi r^{2}}\)

\(d V=i d R=\frac{i \rho d x}{\pi r^{2}}\)

\(E=\frac{d V}{d x}=\frac{i \rho}{\pi r^{2}}\)

\(V_{d}=\frac{e E \tau}{m}\)

\(\therefore V_{d} \propto E\)

\(\rightarrow E \propto \frac{1}{r^{2}}\)

if \(r\) decreases, \(E\) will increase \(: V_{d}\) will increase