‘Offered’ means
‘to present to’. The passage mentions "a discount", you are usually 'offered/presented' a discount'. Hence B is correct.
‘Served’
means ‘to offer food’ or 'to offer service/time'. In the given sentence, the words that follow the blank "a discount" is not a food item, hence the word isn't relevant here and A is incorrect.
‘Preferred’
means ‘to desire one thing over another’; the meaning isn't relevant to the sentence. Hence option C is incorrect.
‘Liked’ means 'to be fond of' or 'to be satisfied with', the word isn't relevant to the sentence. Hence D is also incorrect.