First Principles Thinking: leverage as an ROE driver
B is correct. In the three-way decomposition, ROE = Net profit margin × Total asset turnover × Leverage. With margin and turnover constant, ROE moves proportionally with leverage. Because the company borrows at a rate below what it earns on the borrowed funds, leverage is effective and ROE rises.
A is wrong because the three-way framework holds margin constant here, and the scenario states leverage is effective, so the leverage effect dominates.
C ignores that ROE depends on leverage in addition to ROA; a higher leverage factor raises ROE even when ROA is unchanged.