Foreign Exchange Rate Definition Steps:

• Identify the core concept: Foreign exchange rate involves currencies in international trade.
• Eliminate unrelated options: A relates to trade balance, D to population movement.
• Compare economic terms: Exchange rate specifically prices currencies, not goods.
• Confirm standard definition: It's how much one currency buys of another.

Why B is correct:

• Foreign exchange rate is defined as the price of one currency expressed in terms of another, per economic principles like those in international finance.

Why the others are wrong:

• A: Describes trade balance or current account deficit, not currency pricing.
• C: Refers to barter or relative prices of goods, not currencies.
• D: Involves migration flows, unrelated to economics or exchange.

Final answer: B