Final answer:
The expression f(12) = 500 indicates a pollution removal rate of 500 kg/day from a lake on day 12.
Step-by-step explanation:
The statement f (12) = 500 refers to the rate at which pollution is removed from a lake on day 12. It indicates that on the 12th day, pollution is being cleaned from the lake at a rate of 500 kilograms per day (kg/day). This is a specific instance of the function f(t), which in general tells us the rate of pollution removal on any given day t.