The correct answer is A. 4.55%.
The labour turnover rate is calculated by dividing the number of workers who left the company during a given period by the average number of workers employed during that period. In this case, the number of workers who left is 5 + 20 = 25. The average number of workers employed is (500 + 600) / 2 = 550. The labour turnover rate is therefore 25 / 550 = 4.55%.
Option B is incorrect because it is the percentage of workers who were discharged. Option C is incorrect because it is the percentage of workers who were recruited. Option D is incorrect because it is the percentage of workers who were recruited to fill vacancies.