Ans : (b)
$\lambda = 488\: nm =488 \times 10^{-9} m$
$V_{\circ} = 0.38 V = 0.38/ (1.6 \times 10^{-19} ) eV$
So, $eV_{\circ} = hc/ \lambda – \phi_{\circ}$
$ \Rightarrow \phi_{\circ} = hc/ \lambda - eV_{\circ}$
$= (6.6 \times 10^{-34} \times 3 \times 10^8 )/ (1.6 \times 10^{-19} \times 488 \times 10^{-9} ) –
(1.6 \times 10^{-10} \times 0.38)/(1.6 \times 10^{-19} )$
$= 2.54 – 0.38 = 2.16 \: eV$