Nouns ending in -tion are feminine.
The suffix forms abstract nouns of action or state.
Is population masculine or feminine?
lapopulation
The rule behind it
99.8% of -tion nouns are feminine
In context
Les statistiques montrent que la population mondiale augmente.
Statistics show that the population of the world is increasing.
Le Japon a une densité de population élevée.
Japan has a high population density.
La population urbaine des États-Unis est en augmentation.
The urban population of America is increasing.