70, 71, 76, __, 81, 86, 70, 91. What number should fill the blank?

The answer is $\boxed{\text{80}}$.

The sequence is $70, 71, 76, 80, 81, 86, 70, 91$. It is a simple alternating addition and subtraction sequence with a pattern of $+5, -5, +6, -6, +7, -7$. Therefore, the missing number is $80$.

Option A: $70$ is the first number in the sequence.
Option B: $71$ is the second number in the sequence.
Option C: $80$ is the third number in the sequence.
Option D: $96$ is not in the sequence.