sample or population

I saw following question on Shweser bank. They calculated assuming this as a sample, since I saw variance was divided with n-1. How do you assume it as a sample vs population ------------------------ Annual Returns on ABC Mutual Fund 11.0% 2% 4% 12.5% 8.0% If the risk-free rate was 4.0 percent during the period 1991-2000, what is the Sharpe ratio for ABC Mutual Fund for the period 1991-2000?