Arrange Li, Na, K, Rb, Cs in order of increasing electronegativity:

The correct option is B $Cs<Rb=K<Na<Li$
As we move from top to bottom in a group the $electronegativity$ (measure of the tendency of an atom to attract a bonding pair of electrons) will be decreased because the atomic radius will be increased so distance from outer electrons to the nucleus will be increased, so the electrons cannot be attracted easily.

Li, Na, K, Rb, Cs are IA group elements from Li to Rb the electronegativity will be decreased.

So, the correct order of electronegativity is $Cs<Rb=K<Na<Li$
Hence, option A is correct.