Official Explanation
It is fairly clear that the greatest number of pens that can be bought for $25 will consist mostly, if not entirely, of $2 pens. In fact, it is reasonable to begin by looking at how many of the $2 pens the customer can buy if the customer does not buy any $3 pens. It is easy to see that the customer could buy 12 of the $2 pens, with $1 left over.
If the customer bought 11 of the $2 pens, there would be $3 left over with which to buy a $3 pen. In this case, the customer could still buy 12 pens. If the customer bought 10 of the $2 pens, there would be $5 left over. Only 1 of the $3 pens could be bought with the $5, so in this case, the customer could buy only 11 pens.
As the number of $2 pens decreases, the total number of pens that the customer can buy with $25 decreases as well. Thus the greatest number of pens the customer can buy with $25 is 12.
The correct answer is Choice D