The greenhouse gases are called so because they absorb the long wave infra-red radiations reflected from the earth's surface and trap them such that they do not get reflected back to space. This trapping of the long wave infra-red radiations causes an increase in the temperature of the atmosphere, called global warming. CO$ _{2}$, CH$ _{4}$, N$ _{2}$O, CFCs are the gases that are capable of showing greenhouse effect. These have increased in the concentration over the years due to the anthropogenic activities.
Hence, the correct answer is 'Long wave infra-red radiations'.
What is relative contribution of $CO _2, CH _4$, CFC and $N _2O$ to global warming?
Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere up to a specific time horizon, relative to carbon dioxide. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide and is expressed as a factor of carbon dioxide an index of global warming potential for methane, carbon monoxide, nitrous oxide and CFCs relative to that of carbon dioxide. The major contribution is from CO$ _2$ (60%),CH$ _4$ (20%), CFC and N$ _2$O attributes to 14% and 6% respectively.