Correct Answer - Option 3 : 68 : 54
The pattern followed here is,
For,
84 : 72
(First Digit of the first number) - 1 and half of the second number = 72
(8 - 1) (4 ÷ 2)
72 = 72.
Similarly,
For 77 : 64
(7 - 1) (7 ÷ 2)
6 (3.5)
63.5 ≠ 64.
For 64 : 56
(6 - 1) (4 ÷ 2)
52 ≠ 56
For 68: 54
(6 - 1) 8 ÷ 2
54 = 54
For 76 : 62
(7 - 1) (6 ÷ 2)
63 ≠ 62
Hence, the correct answer is "68 : 54".