Which issue was at the heart of one of the compromises made between northern and southern states during the constitutional convention of 1787?
a. western land claims
b. counting slaves in the population
c. allowing new states into the union
d. regulation of interstate commerce

It was the issue of "b. counting slaves in the population" that was at the heart of one of the compromises made between northern and southern states during the Constitutional Convention of 1787, since the Southern colonies wanted them to count as representation while the Northern colonies were opposed to such a measure.