Questions: The chronicle sources of the ninth century CE state that Norsemen attacked the rest of Europe

The answer is: from ships.

Explanation for each option:

1. Marching on foot from their homelands somewhere to the east: This is incorrect. The Norsemen, also known as Vikings, primarily used their advanced seafaring skills to travel and raid. Their homelands were in Scandinavia (modern-day Norway, Sweden, and Denmark), which are to the north, not the east, of the rest of Europe.

2. As mercenaries fighting for the Umayyad emir of Spain: This is incorrect. While there were instances of Norsemen serving as mercenaries in various parts of Europe, including the Byzantine Empire, their primary mode of attack during the ninth century was not as mercenaries for the Umayyad emir of Spain.

3. From ships: This is correct. The Norsemen are well-documented for their use of longships, which allowed them to navigate both open seas and shallow rivers. This capability enabled them to launch surprise attacks on coastal and riverine settlements across Europe.

4. By riding from the east on horseback: This is incorrect. The Norsemen were not known for their use of cavalry in their raids. Their primary mode of transportation and attack was via their ships, which allowed them to strike quickly and retreat just as swiftly.

In summary, the Norsemen attacked the rest of Europe primarily from ships, leveraging their superior naval technology and seafaring skills.