which metals from K,Na,Cs, Cr undego photoelectic effect easily

Cs

it is the most electro+ metal and has least IE.

Cs

Photoelectric effect deals with ejection of electrons so the its rate depends on how easily an electron can be removed from the outermost shell.

Thus, the element which will have the lowest ionisation enthalpy will eject electrons easily from the metal surface and so photoelectric effect will take place much easily.

In the above metals Cs has lowest ionisation enthalpy i.e 380 kJ/mol

So Cs will undergo photoelectric effect most easily.

It is Cesium(Cs). It is explained in NCERT in 2nd chapter of class 11.