Demonstrative pronoun

Demonstrative pronouns have 3 functions.
1 They replace a noun already mentioned.

C’est ton livre ?
Ce n’est pas mon livre. C’est celui de Priya.

2 Celui-ci and Ceux-là are used whena choice is indicated.
  this one     that one

J’aime ces gâteaux.
Tu veux celui-ci ou celui-la ?

3 They come along with a relative pronoun.
They replace a noun already mentioned and functions
as subject of the verb:
Celui qui , celle qui
the one that, the one who, and

ceux qui , celles qui 
those that / who

Qui est Meena ? C’est celle qui t’a parlé.
Who is Meena?   She is the one that talked to you.

Celle qui replaces the noun already mentioned Meena and
functions as subject.

As object of the verb :

Celui que, celle que
the one that / whom / which,

ceux que, celles que
the ones that / whom / which

Qui est cet homme ? C’est celui que j’aime.
Who is this man?    This is the one I love.

Celui que replaces he noun already mentioned and functions
as object.

As object of the proposition ‘de’
Celui dont, celle dont
the one whose, the one of which,

ceux dont, celles dont 
those whose, the ones of which

De qui tu parles ?             
Tu ne connais pas celui dont je parle
(Who are you talking about?
You don't know the one I am talking about! )

Celui dont is the object of the proposition ‘de’