What is the year that branch opened?
SELECT T2.open_year FROM membership_register_branch AS T1 JOIN branch AS T2 ON T1.branch_id   =   T2.branch_id WHERE T1.register_year   =   2016 GROUP BY T2.branch_id ORDER BY count ( * )  DESC LIMIT 1
year that branch opened refers to T2.open_year; JOIN operation is used to combine rows from two or more tables, based on a related column between them; WHERE clause is used to filter records; GROUP BY statement groups rows that have the same values into summary rows; ORDER BY clause is used to sort the result set in ascending or descending order; COUNT(*) is used to return the number of rows in a group; LIMIT 1 is used to return only the top record based on the ORDER BY clause.