From the given options, select the number that will come next in the following series.

5, 15, 45, 135, 405, ?

Given: 5, 15, 45, 135, 405, ?

Logic: Each number is multiplied by 3 to get the next number.

5 × 3 = 15
15 × 3 = 45
45 × 3 = 135
135 × 3 = 405
405 × 3 = 1215

Thus, the correct option is  (a) 1215.