SAT Question 739f1bbc: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice A is correct. In an equilateral triangle, all three sides have the same length. It's given that in triangle $ABC$, $AB=4,680$ mm and $BC=4,680$ mm. Therefore, if $AC=4,680$ mm, then all three sides of triangle $ABC$ have the same length, so triangle $ABC$ is equilateral. Therefore, $AC=4,680$ mm is sufficient to prove that triangle $ABC$ is equilateral.