Zhang wants to help potential customers make the decision to purchase their product. Which metric will best help Zhang monitor this campaign result?

Advertising Cost of Sales (ACOS) is the best metric because it directly measures the efficiency of ad spend in generating sales. ACOS calculates the percentage of revenue spent on advertising, indicating how much it costs to acquire a sale. A lower ACOS means more efficient spending and a better return on investment, directly aligning with the goal of driving purchases. Store visits are a measure of foot traffic, not online purchases. Impressions indicate how many times an ad was shown, but not whether it led to a purchase.