The correct option is B their
Here, 'their' as Personal Pronoun is correct because it implies of or belonging to them. Here, boys are cleaning shoes that 'belong to them'.
'There' is incorrect as it implies in, at or to that place or position.
'Theirs' is incorrect as it is the possessive form of they and implies of or belonging to them.
'Them' is incorrect because it is used when referring to people, animals or things as the object of a verb or preposition.