An idiomatic expression is a type of informal English where the meaning
of the expression is different than the literal meaning of the
expression.
Option C: When the word 'drop' is placed in the sentence, the idiom 'shop till you drop' is formed. It's an American idiom that means 'buying things until you get physically tired'. It is relevant to the given statement, hence option C is correct.
The words 'dunk' and 'drown' do not complete the meaning of the phrase 'shop till they...'. Also, these words are not relevant to the statement. ('Dunk' either refers to dunking a cookie in milk or a basketball shot. 'Drown' refers to being submerged in water.) Hence options A and D are incorrect.
'The' is an article and does not provide a complete, relevant meaning (shop till they the) and hence B is also incorrect.