Concept:The name of the battle comes from the defensive method the Muslims used to protect Madinah.
Explanation:The Battle of Khandaq took place in the 5th year after Hijrah.
The Quraysh and their allies marched towards Madinah.
Salman al-Farisi advised the Prophet (SAW) to dig a ditch around the city.
This ditch protected Madinah from the enemy forces.
The Arabic word "Khandaq" means a ditch or trench.
Therefore, the battle became known as the Battle of Khandaq because the Muslims dug a ditch to defend the city.
It is also called the Battle of the Confederates.
Answer:The Battle of Khandaq is so called because the Muslims dug a ditch to defend Madinah.
So, the correct option is C.