The warmth felt from a campfire is a result of thermal radiation, a form of heat transfer that does not require a medium. All objects with a temperature above absolute zero emit electromagnetic radiation. For an object at the temperature of a campfire, the peak of this emission spectrum is in the infrared range. This infrared radiation travels through the air, is absorbed by the person's skin, and is perceived as heat. This process is distinct from convection, where heat is transferred by the movement of heated air.

B. Microwaves: Microwaves have longer wavelengths and lower energy than infrared radiation and are not the primary type of thermal energy emitted by a campfire.

C. X-rays: X-rays are very high-energy electromagnetic waves produced by high-energy atomic processes, not by the chemical combustion in a fire.

D. Ultraviolet radiation: While a fire emits some visible and a small amount of ultraviolet (UV) light, the vast majority of its radiant energy is in the infrared part of the spectrum.

1. OpenStax. (2016). University Physics Volume 2. Chapter 1: Temperature and Heat, Section 1.5: Mechanisms of Heat Transfer. Retrieved from https://openstax.org/books/university-physics-volume-2/pages/1-5-mechanisms-of-heat-transfer. (See paragraph 10, which states, "The warmth you get from a fire is transferred to you by radiation... Most of the radiation is in the infrared part of the spectrum.")

2. NASA Science. (n.d.). Tour of the Electromagnetic Spectrum: Infrared Waves. NASA. Retrieved from https://science.nasa.gov/ems/07infraredwaves. (The page states, "In fact, the heat that we feel from sunlight, a fire, a radiator or a warm sidewalk is infrared.")

3. Nave, R. (n.d.). Heat Transfer. HyperPhysics, Georgia State University. Retrieved from http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html. (The section on "Radiation" explains that thermal radiation is electromagnetic radiation and provides examples like the sun and a fire, noting the energy is primarily infrared.)