Which year did Europe produce the most cars?
year refers to model_year; Europe refers to country = 'Europe'; the most cars refers to max(model_year)
SELECT T1.model_year FROM production AS T1 INNER JOIN country AS T2 ON T1.country = T2.origin WHERE T2.country = 'Europe' GROUP BY T1.model_year ORDER BY COUNT(T1.model_year) DESC LIMIT 1