Introduction:
Black body radiation is the electromagnetic radiation emitted by a perfect black body, which absorbs all incident radiation without reflecting or transmitting any. This radiation is characterized by a continuous spectrum of wavelengths, and its intensity at each wavelength is determined solely by the temperature of the black body.

A tungsten lamp:
A tungsten lamp, commonly used in incandescent light bulbs, does not approximate black body radiation well. Although it emits visible light, its spectrum is not continuous and lacks the characteristic shape of black body radiation.

Sodium flame:
A sodium flame, produced by heating sodium in a flame, emits a characteristic yellow-orange light. However, its spectrum consists of discrete lines rather than a continuous spectrum, making it a poor approximation of black body radiation.

Hot lamp black:
Hot lamp black refers to a carbon-based pigment that absorbs most of the incident light, appearing black. While it does emit some thermal radiation, its spectrum is not continuous and does not resemble black body radiation.

A hole in a cavity, maintained at constant temperature:
A hole in a cavity, maintained at a constant temperature, is the best approximation of black body radiation among the given options. This is known as a black body cavity. When the walls of the cavity are heated to a specific temperature and the cavity is perfectly insulated, the radiation inside the cavity approximates black body radiation. The cavity acts as a perfect absorber and emitter of radiation, resulting in a continuous spectrum of wavelengths that depends solely on its temperature.

Conclusion:
Among the given options, a hole in a cavity maintained at a constant temperature best approximates black body radiation. This is because the cavity acts as a perfect absorber and emitter of radiation, producing a continuous spectrum of wavelengths determined solely by its temperature. The other options, such as a tungsten lamp, sodium flame, and hot lamp black, do not exhibit the characteristic continuous spectrum associated with black body radiation.