To solve this question, the user needs to understand the concept of profit percent, cost price and selling price.
Let M be the marked price of each pen
The cost price of 30 pens is equal to the cost price of 27 pens, since the retailer pays equal to the marked price of 27 pens. Therefore, the cost price of each pen is $\frac{M}{30}\times27 = \frac{9M}{10}$.
When the retailer sells each pen at the marked price M, the selling price of each pen is M.
So the profit for each pen is $M - \frac{9M}{10} = \frac{M}{10}$.
Therefore, the profit percent is $\frac{\text{Profit}}{\text{Cost Price}}\times100 = \frac{\frac{M}{10}}{\frac{9M}{10}}\times100 = \frac{1}{9}\times100 = 11\frac{1}{9}\%$.
Therefore, the answer is:
The Answer is: D. 11 1/9%