#### Answer

The $xz$-plane

#### Work Step by Step

$\textbf{r}(t) = t\textbf{i} + t^2\textbf{k}$
The curve $\textbf{r}(t)$ only has $\textbf{i}$ and $\textbf{k}$ components. In other words, the curve only has a $x$ and $z$ components, meaning that the curve must reside in the $xz$-plane.