72, 76, 73, 77, 74, __, 75. What number should fill the blank?

70
71
75
78

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

The sequence is a simple alternating addition and subtraction of 4. Starting with 72, we add 4 to get 76. Then we subtract 4 to get 73. We continue this pattern, adding 4 and then subtracting 4, to get 74, 77, 75.

Option A is incorrect because 70 is not in the sequence.

Option B is incorrect because 71 is not in the sequence.

Option D is incorrect because 78 is not in the sequence.