Final answer:
The issue that bitterly divides Snowball and Napoleon in chapter 5 of 'Animal Farm' is control of the windmill project.
Step-by-step explanation:
The issue that bitterly divides Snowball and Napoleon, and consequently the farm, in chapter 5 of 'Animal Farm' is a) control of the windmill project. Snowball and Napoleon have differing views on the construction of the windmill, with Snowball advocating for its construction and Napoleon opposing it. This issue leads to a power struggle between the two characters and ultimately divides the farm.