Concept:The given sequence is an arithmetic progression (AP).Use the formula for the sum of n terms: Sn=2n[2a+(n−1)d].Explanation:The series is 3,6,9,…First term, a=3.Common difference, d=6−3=3.We need the sum of the first 18 terms, so n=18.Substitute these values into the sum formula.S18=218[2(3)+(18−1)(3)]=9[6+17×3]=9[6+51]=9×57=513.Thus, the sum of the first 18 terms is 513.Answer:The correct answer is 513, which is Option B.