Answer: The population in 1990 or the initial population.
Step-by-step explanation:
Here, the function that shows the population after t years since 1990,

For t = 0,
The population is,

( Since,
)
Since, the population is estimated from 1990,
Thus, t = 0 represents the the year 1990,
⇒ f(0) represents the population in 1990,
⇒ The population in 1990 = 523.