According to theSolution Trainarticle on the Scaled Agile Framework website, Solution Trains are the organizational construct used to build large solutions that require the coordination of multiple ARTs and suppliers. The article states that ''Solution Trains form for different reasons. Some trains form with new ARTs and Agile Teams to specifically address a large initiative, while others begin by combining existing ARTs and teams.'' Therefore, the correct answer is D, when the size and complexity require two or more ARTs. The other options are not relevant to the definition of a Solution Train.