The correct option is A It had a larger pool of available reinforcements and could resupply its troops
The Union was more likely to win a war of attrition because it had a larger pool of available reinforcements and could resupply its troops. A war of attrition is war fought without quarter to exhaust the troops, supplies and resources of the enemy. The South had far fewer of those thing to begin with and so was more likely to lose a war of attrition.