The correct pairings are: Comparative cost theory by David Ricardo (a-3), Opportunity cost theory by Gottfried Haberler (b-1), Factor endowment theory by Heckscher-Ohlin (c-4), and the Doctrine of reciprocal demand by J.S. Mill (d-2). These theories form the core of classical and neoclassical international trade economics.

In the context of the Balance of Payments, the current account and capital account are linked. A deficit in the current account (meaning more money is flowing out for goods and services than is flowing in) must be financed by a surplus in the capital account, which represents net inflows of investment and capital.

Globalization provides firms with access to international markets, allowing them to expand their customer base and increase sales volume. By operating globally, companies can leverage economies of scale, access cheaper raw materials, and utilize diverse talent pools. While it increases competition, the primary strategic benefit is the ability to tap into new demand, which is essential for firms seeking to scale their operations beyond domestic saturation points.

Companies employ various entry modes to enter international markets. Exporting allows for direct sales, licensing and franchising enable brand expansion with lower capital risk, and joint ventures facilitate shared resources and local market knowledge. All these methods are standard strategic approaches for global business expansion.

Speculation involves buying or selling currencies based on anticipated future price movements. This activity often amplifies existing market trends and can lead to increased volatility and violent fluctuations in exchange rates, as speculators react to news and market sentiment, thereby impacting the stability of the currency market.

A flexible exchange rate system allows currency values to adjust based on market supply and demand. If a country faces a balance of payments deficit, its currency typically depreciates, making its exports cheaper and imports more expensive. This mechanism naturally encourages exports and discourages imports, leading to an automatic correction of the trade imbalance without requiring direct government intervention or depletion of foreign exchange reserves.

Mint Parity theory applies to gold standards, not inconvertible paper currency. The Balance of Payment theory was not propounded by Gustav Cassel; he is associated with the Purchasing Power Parity (PPP) theory. Therefore, both statements provided in the prompt are factually incorrect.

The matching is as follows: Importing is a mode of international business (a-1); Marketing represents the functions of international business (b-2); Sales expansion is a primary objective of international business (c-3); and Economic forces are categorized as external influences on business operations (d-4). This alignment correctly maps the operational, functional, and strategic components of global trade.

Socio-cultural factors typically encompass the customs, lifestyles, values, and demographic characteristics of a society that influence consumer behavior. While poverty is an economic condition that affects purchasing power, it is usually classified under economic factors rather than socio-cultural ones. Techniques, demographics, and changing preferences are direct components of the cultural and social landscape that businesses must analyze to adapt their marketing and operational strategies effectively.

Michael Porter's 'Diamond Model' identifies four interconnected determinants that create a nation's competitive advantage: (1) Factor Conditions, (2) Demand Conditions, (3) Related and Supporting Industries, and (4) Firm Strategy, Structure, and Rivalry. These elements work together to create a business environment that fosters innovation and productivity, allowing domestic firms to compete effectively on the global stage.