One US dollar (USD) can buy 1.6598 Canadian dollars (CAD). In the foreign exchange market, the exchange rate is typically represented as a ratio between two currencies, where the base currency (in this case, USD) is always equal to one unit, and the quoted currency (in this case, CAD) is what that one base unit is equivalent to in the other currency. Therefore, a USD/CAD currency rate of 1.6598 implies that one US dollar can buy 1.6598 Canadian dollars. This is a fundamental concept in foreign exchange trading and is crucial for understanding how currency values are determined and fluctuate over time.

Choice B is incorrect. This statement would imply that the Canadian dollar is stronger than the US dollar, which contradicts the given exchange rate. The exchange rate of 1.6598 indicates that one unit of the base currency (USD) can buy 1.6598 units of the quote currency (CAD), not vice versa.

Choice C is incorrect. The given ratio does not represent an appreciation or depreciation in value of a currency but rather it represents how much of one currency can be exchanged for another at a particular point in time.

Choice D is incorrect. Similar to choice C, this option misinterprets what an exchange rate signifies. It does not indicate appreciation or depreciation but rather it shows how much one unit of a certain currency can be exchanged for another.