What happened to Charlemagne's empire after the death of his son?

Responses

The empire was taken over by Vikings.
The empire was taken over by Vikings.

The empire was divided among Charlemagne's three grandsons.

The empire was divided among Charlemagne's three grandsons.,

The empire became the country of France.
The empire became the country of France.

The empire became the country of Germany.

The empire became the country of Germany.