Normal greenhouse effect is essential for sustenance of life on Earth as it has raised the surface temperature of earth by

The correct option is 33 ℃.

To understand why this is correct, let us break down the physics of the Earth's natural greenhouse effect step-by-step:
1. Earth's Temperature Without an Atmosphere: If the Earth had no atmosphere, all the incoming solar radiation absorbed by the Earth's surface would be radiated back into space as infrared radiation. Under these conditions of radiative equilibrium, the average surface temperature of the Earth would be approximately –18 ℃ (or 255 K). At this freezing temperature, most water on Earth would be frozen, making the sustenance of life extremely difficult, if not impossible.
2. The Role of Greenhouse Gases: The Earth's atmosphere contains greenhouse gases such as water vapor, carbon dioxide, methane, and nitrous oxide. These gases are transparent to incoming shortwave solar radiation but absorb the outgoing longwave infrared radiation emitted by the Earth. They then re-emit this thermal energy in all directions, including back toward the Earth's surface. This process is known as the natural greenhouse effect.
3. Actual Average Surface Temperature: Due to this heat-trapping mechanism, the actual average surface temperature of the Earth is raised to a comfortable +15 ℃ (or 288 K), which allows liquid water to exist and supports a diverse biosphere.
4. Calculation of the Temperature Raise: The net temperature increase due to the normal greenhouse effect is the difference between the actual surface temperature with the atmosphere and the theoretical temperature without the atmosphere:
$$\mathrm{Temperature\; Raise}=15℃-(-18℃)=33℃$$

Therefore, the natural greenhouse effect has raised the Earth's surface temperature by 33 ℃, keeping the planet warm enough to sustain life.