The brightness of a source with luminosity L is given by:
[tex]b=\frac{L}{4\pi d^2}[/tex]Suppose that expression represents the original brightness if we increase the distance four times, then the new brightness is given by:
[tex]b^{\prime}=\frac{L}{4\pi(4d^)^2}=\frac{L}{16(4\pi d^2)}=\frac{1}{16}b[/tex]This means that the brightness will decrease by 16 times. Therefore, the answer is D.
