Which country has built an experimental fusion reactor often referred to as an "artificial sun" that reportedly reaches temperatures of around 100 million degrees Celsius, roughly six times the temperature of the core of the real Sun?

Introduction / Context:
Several countries are working on experimental fusion reactors that aim to mimic the nuclear fusion processes taking place in the Sun. These devices can reach extremely high temperatures and are sometimes described in the media as "artificial suns". One such reactor reached temperatures around 100 million degrees Celsius, about six times hotter than the core of the Sun. This achievement attracted global attention and often appears in current affairs questions related to science and technology.

Given Data / Assumptions:

• The question describes an experimental fusion device nicknamed an "artificial sun".
• It notes that this reactor reached approximately 100 million degrees Celsius.
• The options given are Japan, North Korea, China and India.
• We assume the question refers to widely reported experiments carried out on a tokamak style reactor.

Concept / Approach:
The key concept is nuclear fusion research and which country recently reported this high temperature milestone. China Experimental Advanced Superconducting Tokamak (EAST) and other Chinese fusion projects have repeatedly been in the news for achieving temperatures around or above 100 million degrees Celsius. These experiments are sometimes described in news headlines as creating an "artificial sun". By contrast, Japan and India also do fusion research but the specific 100 million degrees Celsius reports most prominently involve Chinese devices, while North Korea is not known for such large scale fusion facilities.

Step-by-Step Solution:
1. Focus on the phrase "artificial sun" and the approximate temperature of 100 million degrees Celsius. 2. Recall news reports about the Experimental Advanced Superconducting Tokamak and other Chinese fusion experiments reaching such extreme temperatures. 3. Recognise that these reports consistently identify China as the country operating this reactor. 4. Compare with options: Japan, North Korea, China and India. 5. Japan has advanced technology programmes but the most widely publicised "artificial sun" stories at 100 million degrees concern China. 6. North Korea and India do not feature in these particular headlines. 7. Therefore, select China as the correct answer.

Verification / Alternative check:
International science news outlets and major newspapers report that Chinese scientists operating the EAST tokamak and related projects achieved electron temperatures of around 100 million degrees Celsius. These stories explicitly use the phrase "artificial sun" to describe the device. They identify the reactor location as in China. While other international collaborations such as ITER in France also aim at fusion, they had not reached the same milestone at the time of these reports. This confirms that China is correctly associated with the described experiment.

Why Other Options Are Wrong:

• Japan has a strong nuclear research programme, but the widely reported "artificial sun" at 100 million degrees Celsius in recent years relates more directly to Chinese experiments.
• North Korea focuses on nuclear weapons programmes and missile testing rather than large scale fusion research and does not operate such a fusion reactor in public scientific literature.
• India participates in fusion research, including involvement in the ITER project, but there are no major news reports that India built an "artificial sun" reaching 100 million degrees Celsius in the way described in the question.

Common Pitfalls:
A common error is to assume that any technologically advanced country, such as Japan or India, might be the correct answer and to guess without recalling specific news reports. Another pitfall is to confuse nuclear fusion research with nuclear fission power plants and general nuclear capabilities. Students should remember that the high temperature "artificial sun" headlines have been particularly associated with China and its tokamak reactors. Fixing that association in memory helps avoid confusion.

Final Answer:
The experimental fusion reactor nicknamed an "artificial sun" that reached about 100 million degrees Celsius was built in China.