Which number should come in place of the question mark (?) in the following sequence?
86, 89, 82, 85, 78, ?

86, 89, 82, 85, 78, ?

Logic: The pattern alternates between adding 3 and subtracting 7.

89 -86 = 3

82 - 89 = -7

85 - 82 = 3

78 - 85 = -7

Following this pattern, the next difference should be adding 3 to 78:

78 + 3 = 81

So, the next number in the sequence is 81.

Thus, the correct answer is (a).