Which correctly names the line in this figure?

B correctly names the line. In A, DEF (or DF) means a segment, which are not infinite like a line. In C, M represents the plane that this line is on, but not the line. In D, E simply represents point E, a point on the line, but can't represent the line itself. EF with an arrow point both sides means it's a line defined by EF extending to infinity in both directions.