# The alkali metal widely used in photoelectric cells is

$\begin{array}{1 1}(a)\;K&(b)\;Rb\\(c)\;Cs&(d)\;Na\end{array}$

Alkali metals used to have low ionisation enthalpies and therefore, the electrons are easily ejected when exposed to light. Among alkali metals, cesium has lowest ionisation enthalpy and hence it can show photoelectric effect to the maximum extent and therefore preferred in photoelectric cell.
The alkali metal widely used in photoelectric cells is Cs.
Hence (c) is the correct answer.
edited Feb 26