This is a strange scenario, and I'm not sure either answer is quite right.
If he had bought the Mini to replace the Audi (and therefore still uses the Audi), "was" would be correct.
However, because he still uses the Audi, he would probably respond with "The last car I bought was an Audi", "The car I bought before that/the Mini was an Audi", or just "it was an Audi".
If this is a test or question, and these are the only two options you have, I would suggest going with "My last car was an Audi"; that's the regular form, but it doesn't exactly fit with this example.
Hopefully that helps.