Something

\begin{enumerate}
\item This type of motion is acceleration, because a parabola indicates acceleration.
\item 
\item \vspace{1 in} 
\item \vspace{1 in} The relationship between $d$ and $t$ is given by
$d \propto t^2$
\item Pulling the cart with a stretched rubber band caused the cart to
accelerate.
%% no idea why this doesn't work... 
%% perhaps \vspace*{} would be better?
\end{enumerate}