Restricted areas, charted with an 'R' followed by a number (e.g., R-4401), denote the existence of what kind of hazard to aircraft?

Why: The Remote Pilot Study Guide describes restricted areas as containing airspace where flight, while not wholly prohibited, is subject to restrictions because of hazardous activities such as artillery firing, aerial gunnery, or guided missiles; penetration without authorization from the using or controlling agency may be extremely hazardous. Under 14 CFR § 107.45, operating in a restricted area requires permission from that agency.

• Wildlife-refuge bird hazards are not the basis for restricted-area designation; restricted areas are established for hazardous military or similar activities, not wildlife concerns.
• Radar interference is not the stated reason for restricted areas in the study guide; the hazard described is physical (munitions/gunnery), not electromagnetic.