Final answer:
Infrared radiation is the type of radiation trapped by the greenhouse effect, as it is absorbed and re-emitted by greenhouse gases in the atmosphere, leading to an increase in Earth's surface temperature.
Step-by-step explanation:
The type of radiation trapped on Earth's surface by the greenhouse effect is infrared radiation. When the Earth receives energy from the Sun, it includes a range of electromagnetic spectra such as ultraviolet rays, visible light, and infrared. The Earth's surface absorbs this energy, emitting it back into the atmosphere as heat in the form of long wave infrared radiation.
Greenhouse gases in the atmosphere are transparent to most of the incoming solar radiation, but they are effective at absorbing the infrared radiation re-emitted by the Earth's surface. These gases then re-emit the heat in all directions, which includes back towards the Earth, trapping the heat and causing the planet to be warmer than it would be otherwise.
In summary, the number '2' represents infrared radiation on the provided list, which corresponds with the type of radiation responsible for the greenhouse effect according to scientific understanding.