Common Core End-of-Course Assessment - Page 879: 33

$\bf{B}$

We need to use the Pythagorean Theorem to compute the length of the segment $x$. That is, $x^2+(0.8)^2=(1)^2$ or, $x^2+0.64=1$ or, $x^2=1-0.64$ or, $x=\sqrt {0.36}=0.6$ Therefore, the correct option is $\bf{B}$.