The correct answer is Option 4: Nicaragua.

Nicaragua is the country that has been planning to construct a rival to the Panama Canal to link the Pacific and Atlantic Oceans. The project is known as the Nicaraguan Canal or the Interoceanic Grand Canal. It aims to provide an alternative route for global maritime trade, bypassing the Panama Canal and reducing shipping distances and costs.

Here are some key points and additional information to support this choice:

1. Nicaraguan Canal Project: In 2013, the Nicaraguan government granted a concession to the Hong Kong Nicaragua Canal Development Investment Company (HKND Group) to build the canal. The project involves the construction of a 278-mile (446 km) waterway that connects the Pacific Ocean to the Caribbean Sea, passing through Lake Nicaragua. The estimated cost of the project was initially around $50 billion, making it one of the largest infrastructure projects in history.

2. Rivalry with the Panama Canal: The construction of a rival canal in Nicaragua is seen as a direct competitor to the Panama Canal. The Panama Canal, opened in 1914, has been a vital maritime route for global trade, connecting the Pacific and Atlantic Oceans. However, the Panama Canal has faced capacity limitations in recent years, leading to congestion and longer wait times for ships. The Nicaraguan government saw an opportunity to capitalize on this and proposed an alternative route to alleviate the strain on the Panama Canal.

3. Economic Benefits: Proponents of the Nicaraguan Canal argue that it could bring significant economic benefits to Nicaragua and the region. The project promises to create jobs, attract foreign investment, and boost economic growth. It could also potentially transform Nicaragua into a major logistics and transshipment hub, attracting shipping traffic and generating revenue from tolls.

4. Environmental Concerns and Challenges: The Nicaraguan Canal project has faced criticism and controversy. Environmentalists and local communities have raised concerns about the potential ecological impact of the canal on Lake Nicaragua, the largest source of freshwater in Central America. There are also concerns about displacement of indigenous communities and the lack of transparency and public consultation throughout the project`s development.

5. Project Delays and Uncertainty: Despite the initial enthusiasm and high-profile announcements, the Nicaraguan Canal project has faced significant challenges and has been plagued by delays. As of my knowledge cutoff in September 2021, no significant progress has been made, and the project`s future remains uncertain. Financial, technical, and environmental hurdles have hindered its implementation.

In conclusion, Nicaragua is the country that has been planning to construct a rival to the Panama Canal to link the Pacific and Atlantic Oceans. However, it is important to note that the project has faced numerous challenges, and its current status and feasibility remain uncertain.