Final answer:
The rate used by commercial banks for currency exchange transactions is typically the spot rate, which reflects the current market value for immediate delivery.
Step-by-step explanation:
The rate charged by commercial banks for the purchase of any foreign currency (in Canadian dollars) on any given day would generally be based on the spot rate. This rate indicates the price at which a currency can be bought or sold with immediate delivery. Unlike the forward rate which is used for trading currencies at a future date, the spot rate is current and reflects real-time market conditions. The average rate and closing rate might be used for accounting or analysis purposes, but it's the spot rate that is typically used for transactions occurring on-the-spot.