The concept of carbon credit originated from the Kyoto Protocol.

Option 1, the Earth Summit, Rio de Janeiro, was focused on sustainable development but did not introduce the concept of carbon credits.

Option 2, the Kyoto Protocol, is the correct answer as it was this international treaty, agreed in 1997, that brought in the concept of carbon credits. It allows countries that have emission-reducing projects to earn carbon credits. These can be traded internationally and used by industrialized countries to meet part of their emission reduction targets under the Kyoto Protocol.

Option 3, the Montreal Protocol, was significant for its role in phasing out the production of numerous substances responsible for ozone depletion, not carbon emission reductions.

Option 4 refers to a G-8 Summit, but the concept of carbon credits did not originate from these summits, which are economic meetings between leading industrial nations.

Thus, it is the Kyoto Protocol that is correctly associated with the origination of carbon credits.