What is it about the king's selling of the Wilks's slaves that most angers the townspeople?

He sells them for too little money.

They do not really belong to the Wilks family.

He separates parents and children in the sale.

The townspeople are mostly abolitionists opposed to slavery.

What angers the townspeople the most when the king is selling the Wilks’ slaves is that:

He separates parents and children in the sale.

Separating the parents and children in the sale is a strategy that they thought of to sell the slaves faster. However, the girls found breaking up the family insensitive - which made them upset. There were also a lot of the townspeople who disapproved at separating the families, but the men were still firm on doing it.

The proper answer is "C. He separates parents children in the sale."

You are referencing a scene from _The Adventures of Huckleberry Finn_ where slaves that once belonged to the now-deceased Peter Wilks are being sold off by the King and Duke. The fact that the slaves are being sold is not what gets people angry. What angers people is that the family of slaves is broken up in order to have the sold faster separately than they would have been had they been sold together as a family.