The function \( f(x) = 500(1.10)^x \) represents the growth of a species of fish over time, where \( x \) is the time in months since January 2019.
In this function, the number 500 is the initial value (the value when \( x = 0 \)). This means it represents the starting number of fish in the lake when the observation begins in January 2019.
Therefore, the correct response is:
D The starting number of fish in the lake.