Emissivity is the property of a material that describes its ability to emit thermal radiation. It is defined as the ratio of the total emissive power of a body to the total emissive power of a black body at the same temperature. The correct answer to the question is option 'D'.

Explanation:

1. Emissivity:
- Emissivity is a dimensionless quantity that ranges from 0 to 1.
- A value of 0 indicates that the material is a perfect reflector and does not emit any thermal radiation.
- A value of 1 indicates that the material is a perfect emitter and emits thermal radiation as a black body.
- Emissivity depends on the material's surface properties, such as roughness, composition, and temperature.
- Different materials have different emissivity values, and they can vary with wavelength and temperature.

2. Total emissive power:
- The total emissive power of a body is the total amount of thermal radiation it emits per unit area and per unit time.
- It is given by the Stefan-Boltzmann law, which states that the total emissive power is proportional to the fourth power of the absolute temperature.
- Mathematically, the total emissive power (P) is given by P = σ * A * ε * T^4, where σ is the Stefan-Boltzmann constant, A is the surface area, ε is the emissivity, and T is the absolute temperature.

3. Black body:
- A black body is an idealized object that absorbs all incident radiation without reflection or transmission.
- It is a perfect emitter of thermal radiation and has an emissivity of 1 at all wavelengths and temperatures.
- The total emissive power of a black body is given by the Stefan-Boltzmann law: P_bb = σ * A * T^4.

4. Ratio of emissive powers:
- The ratio of the total emissive power of a body to the total emissive power of a black body at the same temperature is given by P/P_bb.
- Substituting the expressions for P and P_bb, we get P/P_bb = (σ * A * ε * T^4) / (σ * A * T^4).
- The surface area and the Stefan-Boltzmann constant cancel out, leaving us with ε.
- Therefore, the ratio is equal to the emissivity of the body.

Conclusion:
The ratio of total emissive power of a body to the total emissive power of a black body at the same temperature is equal to the emissivity of the body. Hence, the correct answer is option 'D' - Emissivity.