The correct answer is option 'C' which states that according to the Kyoto Protocol, the major nations were required to reduce the concentration of greenhouse gases by 2012. Let's delve into the details of why option 'C' is the correct answer.

The Kyoto Protocol, adopted in 1997, is an international treaty aimed at combating climate change by reducing greenhouse gas emissions. It sets binding targets for the reduction of greenhouse gas emissions for developed countries. These targets are known as "quantified emission limitation and reduction objectives" (QELROs).

Under the Kyoto Protocol, developed countries are divided into two groups: Annex I and non-Annex I countries. Annex I countries are the major industrialized nations, including countries like the United States, Canada, Japan, and members of the European Union. Non-Annex I countries are developing nations that have fewer obligations in terms of emission reductions.

The Kyoto Protocol set different targets for each Annex I country, based on their historical emissions and their capacity to reduce emissions. The overall goal of the protocol was to reduce the collective emissions of Annex I countries by at least 5% below 1990 levels during the period 2008-2012.

The correct answer, option 'C', indicates that the major nations were required to achieve their emission reduction targets by 2012. This means that by the end of 2012, these countries were expected to have taken significant steps to reduce the concentration of greenhouse gases.

It is important to note that the Kyoto Protocol had a limited scope as it only covered a specific set of countries and did not include major emitters such as China and India. Additionally, the effectiveness of the protocol has been a subject of debate, as some countries did not meet their targets and there were no clear penalties for non-compliance.

Nonetheless, the Kyoto Protocol played a crucial role in raising awareness about the need for global action on climate change and establishing a framework for international cooperation. It paved the way for subsequent climate agreements and negotiations, such as the Paris Agreement in 2015.

In conclusion, option 'C' is the correct answer as it accurately states that according to the Kyoto Protocol, the major nations were expected to reduce the concentration of greenhouse gases by 2012.