Final answer:
The most influential factor for a nation in deciding the quantity of oil to purchase from a particular site is the current market price of oil.
Step-by-step explanation:
When deciding the quantity of oil to purchase from a particular site, a nation is likely to be most influenced by the current market price of oil. The market price of oil reflects supply and demand dynamics, including factors such as global production, geopolitical events, and economic conditions. Nations often consider the affordability of oil and its impact on their economy when making purchasing decisions.