The correct answer is A. 28%.
The ROI of a company is calculated by dividing its net profit by its total assets. In this case, the company has a net profit of 8% of its turnover, which is 0.08 * 4 = 3.2. The company’s total assets are equal to its paid-up capital, which is 5000 * 10 = 50000. Therefore, the ROI of the company is 3.2 / 50000 = 0.0064 = 28%.
Option B is incorrect because it is the ROI of a company with a turnover of five times and a margin of 8% on sales. Option C is incorrect because it is the ROI of a company with a turnover of four times and a margin of 10% on sales. Option D is incorrect because it is the ROI of a company with a turnover of four times and a margin of 12% on sales.