The correct option is B Interested
Let’s examine the sentence with number 31: “Those interesting should register themselves until 10th June.” The verb “interesting” is used to indicate objects that have some features of interest or attraction. To express one’s interest in an event, the verb “interested” is used. So, to correct the sentence, “interesting” at number 31 would be replaced by the verb “interested.” The correct sentence would be, “Those interested should register themselves until 10th June.” Hence, Option A is the correct answer.