You forgot to put the rest of the problem,
Answer:
c The longhorns had been skittish, but Johnny Chillers—who was named for the old Chillers trail—sang to them whenever they started to spook to calm them down.
Explanation:
The best revision that corrects Sam's error in the use of dashes to set off a clause is option C because it correctly separates the group of words which indicates a pause to emphasize that what Johnny Chillers was named after.
An em dash is used to show a pause and are also used to separate groups of words and their usage can be interchanged with commas or hyphens.
