Click on a island only if there is at least one more island that belongs (partially or totally) to the same country as the island you want to click. But be careful, if you click on an island without a pair, you lose instantly. Good luck!
For example, if the answers are A, B, and C; with A and C belonging to the same country, then you should click A and C, but not B.
Watch out for territories, as they are counted as part of their respective countries!