The sides of a real triangle satisfy the triangle inequality, which just says any two sides add up to more than the third. It's strictly more; if two sides add up to exactly the third that's called a degenerate triangle. It's not a real triangle because the vertices are three collinear points.
The triangle equality fails when the sum of the two smallest sides fails to exceed the the largest side.
A. 4+6=10, degenerate triangle, not a real triangle
