#### Answer

C

#### Work Step by Step

Use mental math to quickly find the correct answer, as time is critical when taking a standardized test.
You can see that $4y^4$ is a perfect square that can be factored out of $12y^5$, so the correct answer will contain $\sqrt{4y^4}=2y^2$. Only response C can be correct.
Use the multiplication property of square roots to solve.
$\sqrt{12y^5}=\sqrt{4y^4\times3y}=\sqrt{4y^4}\times\sqrt{3y}=2y^2\sqrt{3y}$