The sum of the first n terms of a series is S_n = 2^n - 1. The series is:
Correct: A
Given S_n = 2^n - 1. The nth term t_n = S_n - S_{n-1} = (2^n - 1) - (2^{n-1} - 1) = 2^n - 2^{n-1} = 2^{n-1}. So the terms are: t_1 = 2^{0} = 1, t_2 = 2^{1} = 2, t_3 = 2^{2} = 4, t_4 = 2^{3} = 8, ... This is a geometric progression with first term 1 and common ratio 2. Therefore, the series is a GP with ratio 2.