Step 1: Understanding the Concept:
International trade occurs through various modes, but a "gateway" implies the primary point through which the bulk of global commodities enter or exit a nation.
Step 2: Detailed Explanation:
1. Sea Ports: Over $90%$ of international trade by volume and $70%$ by value is carried out through maritime transport. Ports are the specialized facilities where sea routes meet land routes for loading and unloading cargo.
2. Others: While airports handle high-value, low-volume goods, and highways handle regional trade, they do not manage the massive volume of global bulk commodities that ports do.
Step 3: Final Answer:
Due to their role in facilitating the majority of world trade, Ports are technically called the 'Gateways of International Trade'. Thus, (B) is correct.