What does a solid triangle indicate on an en route IFR chart?

A solid triangle on an en route IFR chart signifies a compulsory reporting fix. This is a location where an aircraft is required to report its position to air traffic control to ensure that the aircraft's path is monitored accurately. These reporting fixes are crucial for the management of the airspace, particularly in controlled airspace where precise communication with air traffic control is necessary to maintain safe separation between aircraft.

Compulsory reporting fixes are typically placed at strategic points along airways and routes. Their depiction as solid triangles makes them easily recognizable to pilots, ensuring they understand where they need to communicate their position. This enhances situational awareness for both the pilots and air traffic controllers, thereby contributing to overall flight safety.

Other options might refer to important points or features on the chart but do not carry the same requirement for position reporting as the compulsory reporting fixes do.