Which inequality can be used to explain why these three segments cannot be used to construct a triangle? ac + ab > cb ac + cb < ab ac + cb > ab ac + ab < cb

Depending on what the three segments are, I believe you are looking for the Triangle Inequality Theorem? It states that for a triangle to exist it has to follow 3 specific inequalities. A + B > C ; A + C > B , B + C > A. If they all work, then a triangle can be formed from the three segments.