Concept:The nth term of an arithmetic progression is given by the formula Un=a+(n−1)d.Explanation:From the sequence 11,4,−3,…, the first term is a=11.The common difference is d=4−11=−7.Substitute these values into the formula.Un=11+(n−1)(−7)Expand the bracket: (n−1)(−7)=−7n+7.So, Un=11−7n+7.Combine the constant terms: 11+7=18.Thus, Un=18−7n.Answer:Un=18−7n, which is option C.