Which of the following elements has maximum photoelectric effect?
A. Li
B. Na
C. K
D. Cs

Hint- In order to find the element which has maximum photoelectric effect, it is very necessary for us to know about the photoelectric effect then we will proceed further according to the ionization potential of the given element. Finally we will compare different elements given in the options to select the final answer.

Complete answer:
Photoelectric effect - When light shines on metal, electrons can be ejected from the surface of the metal in a phenomenon known as the photoelectric effect. This process is also often referred to as photoemission.
It will be easier for elements with lower work function to show photoelectric effect as there will be less energy requirement to extract the electrons. Also we know that the work function of a metal is directly proportional to the ionization enthalpy of the metals. So lower the ionization enthalpy, lower the work function and lower will be the energy required extracting the electron and consequently it will be easier to show photoelectric effect.
As we move down the group the ionization enthalpy of the element decreases so, maximum photoelectric effect will be shown by the lower member of the group.
On moving down the group of alkali metals, the ionization potential decreases. Cs has maximum photoelectric effect as it has minimum ionization potential (or work function).
Hence, Cs has maximum photoelectric effect.

So, option D is the correct option.

Note- Ionization energy or ionization energy, denoted Eᵢ, is the minimum amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated neutral gaseous atom or molecule. The electrons that are ejected from the metal showing photoelectric effect are called photoelectrons. In terms of their behavior and their properties, photoelectrons are no different from other electrons