Answer
$\text{radian}$
Work Step by Step
Since $\text{arc length}=\text{radius}\times\text{central angle}$ where the central angle is measured in radians, then
$\text{central angle}=\frac{\text{arc length}}{\text{radius}}=\frac{r}{r}=1\text{ radian}$
Hence, the missing expression in the given statement is $\underline{\text{radian}}$.
