Whichever google decides to send you to when you make your query. There is no specific API datacenter.
wait, ur implying that they send u the result from a random datacentre? how do they then decide which one to send u? I mean, if there isnt any consistency then theres little point in using the API as the results would be random :/
The API works in much the same way as the normal google. When you make a query you are routed to a datacenter depending on your location and which servers have the least load at that time.
Datacentre are just the servers where your quaries are solved. Your quaries are sent to the datacenter depending on your location. I have found out a new datacentre tool here http://www.serpanalytics.com/tools/dc