The triangle inequality theorem states that the sum of the lengths of any two sides of a triangle is greater than the length of the third side.

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The main idea being tested is that in any triangle, the sum of the lengths of any two sides is greater than the length of the remaining side. This is true because if you take two sides that share a vertex, the straight distance between the two far endpoints—the third side—must be shorter than going along those two sides in sequence. So the sum of those two side lengths is always strictly larger than the third side, and this applies to all three pairs of sides. That’s what ensures a nondegenerate triangle exists. If a + b were not greater than c, you wouldn’t be able to close the figure into a triangle at all. The statement holds for every triangle, not just special types, which is why the true answer is the best choice.