The correct answer is A. 13%.
The gross profit ratio is calculated by dividing the gross profit by the sales revenue. The gross profit is the difference between the sales revenue and the cost of goods sold.
In this case, the selling price is fixed at 25% above the cost. This means that the sales revenue is 125% of the cost. The cost of goods sold is 100% of the cost. Therefore, the gross profit is 25% of the sales revenue.
The gross profit ratio is 25% / 125% = 13%.
Option B is incorrect because it is the cost of goods sold as a percentage of the sales revenue. Option C is incorrect because it is the gross profit as a percentage of the cost of goods sold. Option D is incorrect because it is the sales revenue as a percentage of the cost of goods sold.