Answer: D. 5 < AC < 15
===========================================================
Explanation:
AB = 10 and BC = 5 are two sides of triangle ABC. The third missing side is AC. We don't have enough information to determine the exact length of AC, but we can narrow things (a bit) to get a range of values.
Through a modified version of the triangle inequality theorem, if we had sides a = 10 and b = 5, then the third side c is restricted like so
a-b < c < a+b
10-5 < c < 10+5
5 < c < 15
This then means that 5 < AC < 15 is the compound inequality that tells us the range of values for side AC. It says that AC is between 5 and 15 units long. Side AC cannot be 5 units exactly, and it can't be 15 units exactly either. I recommend cutting out slips of paper to try this out yourself, and you'll see it's not possible to have something like AC = 15.
So that's why choice D is the answer. Choice A is true, because 10 is between 5 and 15, but it's not the only possible value for AC. There's more to the story. That's why I'm thinking choice D is the better answer. It encapsulates all possible lengths of AC.