Which of the following is considered an observable short-term effect of radiation exposure?

Erythema is the correct answer because it is a visible skin reaction resulting from radiation exposure, characterized by redness. This condition reflects the immediate effects of radiation, particularly at doses that can cause tissue damage. Observable effects, such as erythema, typically occur shortly after exposure at higher levels and are directly related to the damage to the skin's blood vessels and underlying tissues.

Fatigue and nausea, while they can be experienced after radiation exposure, are more indirect and subjective effects that may not be readily observable by someone other than the affected individual. Hair loss can occur due to radiation exposure, but it generally represents a longer-term effect as it usually doesn't manifest until several weeks post-exposure after hair follicles are affected by the radiation. Thus, erythema distinctly stands out as an immediate, observable effect.