Answer:
The answer is A. Yes, Cindy should hire 12th worker.
Explanation:
Please see the below for detailed calculations and explanations:
By increasing one employees, Cindy's cupcakes shop marginal cost per day will increase by the amount equals to the salary and benefit of the 12th in one day which is: $100;
The benefits brought back to Cindy's shop is the increase in marginal revenue of $150 ( calculated as $2,750 - $2,600).
As marginal revenue is higher than marginal cost in case the 12th employees is hired, Cindy should hire one more employees as it will increase her total profit at the end of the day by $50 ( i.e Marginal revenue - Marginal cost = 150 - 100 =$50).