An arithmetic sequence is a sequence of numbers in which each term after the first is obtained by adding a fixed number, called the common difference, to the previous term. For example, the sequence 15, 12, 9, 6, … is an arithmetic sequence because each term is obtained by subtracting 3 from the previous term, which is the common difference. On the other hand, the sequence 15, 30, 60, 120, … is not an arithmetic sequence because the difference between each consecutive pair of terms is not constant. The same is true for the sequences 1, 4, 9, 16, … and 1, 8, 27, 64, …. Therefore, the only arithmetic sequence among the given options is 15, 12, 9, 6, ….