There are two function values equal to 1: (1/a, 1)(a, 1). In the title, the value range is (0, 1). Then, the domain should include at least one of two intervals (1/a, 1) or (1, a). And a> 1; Therefore, 1- 1/a is less than a-1; Therefore, the minimum length interval is (1/a, 1). So 1- 1/a=5/6. We can get a=6.
Answer b, over
If the minimum length interval is mistaken for (1, a), then you have chosen the wrong C.
Finally, I need to explain: I personally think that "if the minimum length of interval [m, n] is 5/6" in the title should be changed to "if the minimum length of interval [m, n] is 5/6".