The given answer, "λειτουργικά συστήματα" (operating systems), is correct because the question asks for something that both MS-Dos and Linux are, while excluding "προγράμματα ζωγραφικής" (painting programs) and "αριθμομηχανές" (calculators). Both MS-Dos and Linux are operating systems, which are software that manages computer hardware and software resources and provides common services for computer programs.