b
According to Einstein's photoelectric equation maximum kinetic energy of photoelectrons,

\(K E_{\max }=E_{v}-\phi\)

or \(2=5-\phi \quad \therefore \phi=3 \mathrm{eV}\)

When \(E_{v}=6 \mathrm{eV}\) then

\(KE_{\max }=6-3=3 \mathrm{eV}\)

or \(e\left(V_{\text {cathode }}-V_{\text {anode }}\right)=3 \mathrm{eV}\)

or \(V_{\text {cathode }}-V_{\text {anode }}=3 \mathrm{V}=-V_{\text {stopping }}\)

\(\therefore V_{\text {stoooing }}=-3 \mathrm{V}\)