SQL> create table bbc(region varchar2(10), name varchar2(10), population number(3))
  2  /

Table created.

SQL> insert into bbc values('a1', 'b1', 100)
  2  /

1 row created.

SQL> insert into bbc values('a1', 'b2', 50)
  2  /

1 row created.

SQL> insert into bbc values('a2', 'c1', 30)
  2  /

1 row created.

SQL> select region, name, population from bbc a where (region, population) in (select region, max(population) from bbc group by region)
  2  /

REGION     NAME       POPULATION
---------- ---------- ----------
a1         b1                100
a2         c1                 30

SQL>