The correct answer is option D, the Second International.

Explanation:
The Second International was an international organization that aimed to promote socialism and coordinate the efforts of socialist parties and labor unions across different countries. It was established in 1889 and existed until the outbreak of World War I in 1914.

Establishment and Purpose:
- The Second International was established during a time of growing socialist movements across Europe and other parts of the world.
- Its primary purpose was to promote the interests of the working class, advocate for workers' rights, and encourage international solidarity among socialist parties and labor unions.
- The organization aimed to achieve social and economic reforms through democratic means.

Membership and Structure:
- The Second International included various socialist and labor parties from different countries.
- It had a decentralized structure, with each member party maintaining its own autonomy and decision-making power.
- The organization held international congresses periodically, where representatives from member parties would gather to discuss common goals and strategies.

Activities and Achievements:
- The Second International played a crucial role in advocating for workers' rights, including the establishment of the eight-hour workday and improved working conditions.
- It also promoted the idea of international solidarity and cooperation among socialist parties and labor unions.
- The organization supported workers' strikes and protests, as well as campaigns for universal suffrage and social reforms.
- Additionally, it facilitated the exchange of ideas and experiences among member parties, helping to shape socialist ideologies and strategies.

Decline and Legacy:
- The outbreak of World War I in 1914 led to the disintegration of the Second International, as member parties took different positions regarding the war.
- The organization failed to prevent the war, and its unity was shattered by nationalistic sentiments.
- However, the Second International's legacy continued to influence socialist and labor movements in the following decades.
- It provided a platform for international cooperation and solidarity among socialists, and its ideas and principles laid the foundation for future socialist and social democratic movements.

In conclusion, the Second International was a significant international organization that aimed to promote socialism and coordinate the efforts of socialist parties and labor unions. It played a crucial role in advocating for workers' rights and promoting international solidarity among socialist movements. While it faced challenges and eventually disbanded during World War I, its legacy continued to shape socialist and social democratic movements in the years to come.