What was the subject of the most debate between large and small states at the Constitutional Convention?

The correct answer is C. Representation.

At the Constitutional Convention, the most heated debate between large and small states was centered around how states would be represented in the new Congress. Large states wanted representation based on population, while small states argued for equal representation regardless of size. This disagreement led to the Connecticut Compromise, which established a bicameral legislature with both proportional representation in the House and equal representation in the Senate.