Answer:
B. He saw the old flag
Step-by-step explanation:
The poem reads, "Under his slouched hat left and right
He glanced: the old flag met his sight.
“Halt!”— the dust-brown ranks stood fast.
“Fire!”— out blazed the rifle-blast."
right before he says "Halt!" in the sentence before it says, "He glanced: the old flag met his sight." Therefore what happened before he yelled, "Halt!' was that he saw the old flag.