The International Thermonuclear Experimental Reactor (ITER) project is planned to be built in Option 2: Southern France. Here`s the reasoning and additional information to support this answer:

1. ITER`s Host Agreement: In 2005, the ITER Members (China, the European Union, India, Japan, South Korea, Russia, and the United States) signed a Host Agreement to establish the ITER project. According to this agreement, the ITER site was designated to be in Cadarache, Southern France. The agreement was reached after a careful evaluation of various potential host locations.

2. Cadarache`s Selection Criteria: The selection of Cadarache, France, as the site for ITER was based on several key criteria:

a) Geological Stability: Cadarache was deemed geologically stable, with a low risk of seismic activity or natural disasters that could potentially threaten the project.

b) Availability of Land: The site offered ample space to accommodate the large-scale ITER complex, including the reactor, research facilities, and necessary infrastructure.

c) Infrastructure and Expertise: France had pre-existing nuclear infrastructure and expertise, making it well-suited to support the construction and operation of the ITER project.

d) Climate: Southern France enjoys a favorable climate for the project, providing suitable conditions for construction and operation.

3. Progress and Construction: Since the selection of Cadarache, significant progress has been made on the ITER project. Construction began in 2010, and as of my knowledge cutoff in September 2021, the project was still underway. The ITER site in Southern France has been actively preparing and constructing the necessary infrastructure for the fusion reactor.

4. European Union`s Role: The European Union plays a significant role in the ITER project. It represents the European Atomic Energy Community (Euratom) and is responsible for coordinating European contributions to ITER. As part of its commitment, the European Union chose to build ITER in Southern France, which falls within its territory.

In conclusion, the International Thermonuclear Experimental Reactor (ITER) project is planned to be built in Southern France, specifically in Cadarache. The site`s selection was based on various factors, including geological stability, availability of land, infrastructure and expertise, and a favorable climate. Construction has been ongoing since 2010, and the European Union, representing Euratom, plays a key role in coordinating the project.