In world transport and infrastructure geography, this question asks you to identify in which European country the world's largest and deepest railway tunnel, known as the Gotthard Base Tunnel, is located.

Introduction / Context:
Mega infrastructure projects such as long tunnels and bridges often feature in current affairs and geography questions. The Gotthard Base Tunnel is especially important because it is one of the longest and deepest railway tunnels in the world, built to ease travel through the Alps. Knowing which country built and hosts this tunnel is a standard part of modern world GK.

Given Data / Assumptions:
The structure mentioned is the Gotthard Base Tunnel.
It is described as the world's largest and deepest rail tunnel.
The options are Belgium, Austria, Switzerland, and Sweden.
We assume the question is about the main country whose territory contains the tunnel and which financed and operates it.

Concept / Approach:
The Gotthard Base Tunnel runs under the Swiss Alps and is part of Switzerland's New Rail Link through the Alps project. Therefore, it is a Swiss infrastructure landmark. Although Austria has many alpine tunnels and is sometimes confused with Switzerland in such questions, careful reading of current affairs material makes clear that this specific tunnel is in Switzerland.

Step-by-Step Solution:
Step 1: Recall that the Gotthard Base Tunnel is built under the Alps as a north south rail link. Step 2: Remember that the country leading this project is Switzerland, which is located in the heart of the Alps. Step 3: Check the answer choices and identify Switzerland among them. Step 4: Eliminate Belgium and Sweden, which have no Alps region and are not connected with this tunnel. Step 5: Eliminate Austria for this specific project, even though it also has alpine tunnels, because the name Gotthard is associated with a Swiss pass and tunnel system.

Verification / Alternative check:
News reports and official documents on the inauguration of the Gotthard Base Tunnel describe Swiss political leaders and European Union officials attending the opening ceremony in Switzerland. Maps of the rail route show the tunnel lying entirely within Swiss territory, connecting the northern and southern parts of the country. These consistent references confirm Switzerland as the correct answer.

Why Other Options Are Wrong:
Belgium: A low lying Western European country with no Alpine region; it does not host the Gotthard Base Tunnel.
Austria: Does have Alpine tunnels but is not the location of the Gotthard Base Tunnel; that tunnel is specifically tied to the Swiss Gotthard massif.
Sweden: A Nordic country known for forests and lakes, but without the Alpine mountain chain and not associated with this rail link.

Common Pitfalls:
The main confusion arises between Switzerland and Austria because both are Alpine countries and both invest heavily in tunnel infrastructure. Candidates who only partly remember the fact may guess Austria. To avoid this, connect the name Gotthard with Switzerland in your memory and recall that Switzerland has also built other major base tunnels like the Loetschberg Base Tunnel.

Final Answer:
The Gotthard Base Tunnel is located in Switzerland.