Explanation:

Let’s analyze the series: 76, 78, 73, 82, 66, 91

- Let’s check the differences between numbers:

- 78 - 76 = 2

- 73 - 78 = -5

- 82 - 73 = 9

- 66 - 82 = -16

- 91 - 66 = 25

- The differences are: +2, -5, +9, -16, +25

Notice the pattern: 2, -5, 9, -16, 25

All these are +[square numbers] and -[square numbers]:

+2(+1²), -5(-2²+1), +9(+3²), -16(-4²), +25(+5²)

- But we see 2, 5, 9, 16, 25 which are 1²+1, 2²+1, 3², 4², 5², pattern breaks at second term.

There is a jump in pattern after the first increase. If we replace 78 by 77:

77-76 = +1 (1²)

73-77 = -4 (–2²)

82-73 = +9 (3²)

66-82 = –16 (–4²)

91-66 = +25 (5²)

- Now it fits the pattern: +1², -2², +3², -4², +5²

- So, the wrong number is 78. Correct pattern needs 77 instead.

Option:2, 78

Option 2 is correct.