Final answer:
The most appropriate word to fill in the blank is 'indicative,' as it denotes that the bustling city center marketplace is a clear sign of the success of international trade.
Step-by-step explanation:
Nothing is more indicative of the success of international trade than the thriving marketplace in the center of the city. This sentence implies that a thriving central marketplace is a clear sign, or indication, of successful international trade. Major cities such as New York, which is home to The New York Stock Exchange, exemplify this through their competitive advantages in commerce and industry stemming from various situation factors, including their central locations relative to other marketplaces. This central marketplace serves as a hub for economic activity, underlining the importance of trade and the exchange of goods and services which are key components of a market economy.