the topic and the author's position
At least according to my classes. It is taught differently in different schools
A or d because there can't just be random commas
B. he treated john more harshly than he treated the other children
The comma here is used to intensify the love/adoration of John for this dog. The fact that 2 adjectives were used to describe the dog, requested the comma to be added, therefore the comma and the adjectives form a "relation" , symbiosis in order to intensify john's feelings for the sheep dog.
