First, evaluate the given function at x=3 to find the amount of bacteria left after 3 seconds:
[tex]\begin{gathered} f(x)=6000(0.5)^x \\ \\ \Rightarrow f(3)=6000(0.5)^3=6000(0.125)=750 \end{gathered}[/tex]There are 750 bacteria left after 3 seconds. Since the initial amount of bacteria was 6000, subtract 750 from 6000 to find the amount of bacteria that has been removed:
[tex]6000-750=5250[/tex]Therefore, the amount of bacteria that has been removed after 3 seconds is 5250.
The correct choice is Option D) 5250.