Based on the information provided regarding the Missouri Compromise and its implications, the most fitting response to why some Southerners might feel that the Missouri Compromise favored the North is:
There was more territory north of the line.
This response highlights the fact that the area north of the Missouri Compromise line (36°30' North) included a significant amount of territory that was designated as free, which could potentially lead to an increase in free states and a political imbalance favoring the North.