Is the region that centers on the islands of the tropical pacific ocean.

Oceania is the region that centers on the islands of the tropical Pacific Ocean.

Explanation:

The Pacific Ocean being the largest ocean in the world owns both east-west and north-south expanse and extends into both tropical and temperate zones of the planet.

The Pacific islands that fall in the tropical belt of the ocean are thus called tropical islands. All the tropical islands in the Pacific Ocean are collectively called as 'Oceania'. It is made up of almost 10,000 small and big islands.

a and b because with the other three, they simplify to something else.