Answer: C. Atom economy is set by the balanced equation and compares the mass of desired product with the total reactant mass. A pathway that incorporates a greater proportion of reactant mass into ethanol therefore has greater atom economy.
A: recovering more ethanol increases the experimental yield, not the atom economy.
B: reducing unreacted bromoethane changes conversion, while the reaction stoichiometry remains unchanged.
D: a catalyst changes the reaction rate, not the stoichiometric mass fraction forming ethanol.
Remember: atom economy depends on the reaction equation; percentage yield depends on the amount actually obtained.