What will come in place of question mark in the following number series ?

Answer & Explanation

Option: [D]

The pattern is

2¹ - 1 = 1

2² - 1 = 3

2³ - 1 = 7

2⁴ - 1 = 15

2⁵ - 1 = 31

2⁶ - 1 = 63

2⁷ - 1 = 127

So, 31 will replace the question mark.