Read the sentence given below to find out whether there is any grammatical error in it. The error, if any, will be in one part of the sentence. The corresponding letter of that part is the answer. If the sentence is error-free then, choose “No error” as the answer. (Ignore the errors of punctuation, if any.)

The store offered a (A)/ discount, only but to members (B)/ of its loyalty program, encouraging (C)/ more customers to sign up. (D)

Correct option is B

The error in the sentence is in part (B). The phrase "only but" is incorrect. It should be "but only." The correct phrase is "but only," which sets a specific condition for who receives the discount.
Corrected sentence: The store offered a discount, but only to members of its loyalty program, encouraging more customers to sign up.