Photoelectric effect: When the light of a sufficiently small wavelength is incident on the metal surface, electrons are ejected from the metal instantly. This phenomenon is called the photoelectric effect.
Explanation
In the given options
Sodium, lithium, and magnesium are electropositive and they also lose electrons easily but their reactivity is much less than Caesium.
Cesium is the most electropositive element of all so it has the minimum ionization energy and so contains the maximum capacity to lose electrons.