Answer
$3$
Work Step by Step
From the graph, the curve is increasing, then decreasing and again increasing as the value of $x$ increases.
So, there are $2$ turning points.
$\begin{align}
& \text{Smallest}\ \text{degree}\ \text{of}\ \text{polynomial}=\left( \text{Number}\ \text{of}\ \text{turning}\ \text{point} \right)\text{+1} \\
& =2+1 \\
& =3
\end{align}$
Hence, the smallest degree of the polynomial is $3$.
