A poultry farmer records the following mortality data over a month: 20 birds died out of a flock of 500. What is the mortality rate of the flock?

Mortality rate is calculated as (number of deaths ÷ total population) × 100. Therefore, (20 ÷ 500) × 100 = 4%. Option A (2%) results from dividing by 1000, option C (5%) is a common overestimate, and option D (10%) is incorrect arithmetic.