Provide the ID of the most popular platform in Europe.
ID refers to game_platform_id; the most popular refers to max(num_sales); in Europe refers to region_name = 'Europe'
SELECT T.game_platform_id FROM ( SELECT T1.game_platform_id, SUM(T1.num_sales) FROM region_sales AS T1 INNER JOIN region AS T2 ON T1.region_id = T2.id WHERE T2.region_name = 'Europe' GROUP BY T1.game_platform_id ORDER BY SUM(T1.num_sales) DESC LIMIT 1 ) t