Concept:Salman al-Farisi is remembered for suggesting a defensive trench strategy to protect Medina during battle.Explanation:At the Battle of Khandaq, the Muslims were unsure how to defend Medina against the Quraish and their allies.Salman al-Farisi proposed digging a trench along the open routes through which the enemy could enter the city.This trench prevented the enemy cavalry from attacking Medina directly.So, Salman al-Farisi played a vital and decisive role in the Battle of Khandaq, also known as the Battle of the Trench.Answer:C. Khandaq