If a : b = b : c, then a4 : b4 is equal to = ?

Answer: Option B

$$\begin{array}{rl}& a:b=b:c\\ & \frac{a}{b}=\frac{b}{c}\\ & b^2=ac\\ & b^4=a^2{c}^2\\ & \therefore a^4:b^4=\frac{a^4}{b^4}\\ & =\frac{a^4}{a^2{c}^2}\\ & =\frac{a^2}{c^2}\\ & =a^2:c^2\end{array}$$