Purchasing options on a basket of currencies rather than an individual currency is the most effective strategy for a multinational corporation to minimize hedging costs. This is because the corporation is exposed to several currencies each month, and by buying options on a collection (basket) of currencies, it can distribute the risk, thereby minimizing it. This strategy allows the corporation to hedge against the risk of multiple currencies simultaneously, which is more cost-effective than hedging against each currency individually. Furthermore, this approach provides a more diversified risk profile, which can help to further reduce potential losses.

Choice A is incorrect. Purchasing options on each currency in the market would not be a cost-effective strategy for the corporation. This approach would require significant resources and may expose the corporation to unnecessary risks associated with individual currency fluctuations.

Choice C is incorrect. Buying an option on a single exposure that applies in one time period (like one month) may not provide adequate coverage for the corporation's foreign exchange transactions, especially if these transactions occur over multiple time periods or involve multiple currencies.

Choice D is incorrect. While buying an option on a single exposure that applies several months might provide some level of protection, it does not necessarily minimize costs associated with hedging as it does not take into account potential changes in currency values over time.