Answer
The angular acceleration $\alpha$ is negative.
Work Step by Step
$\alpha = \frac{d\omega}{dt} = \frac{d^2\theta}{dt^2}$
If the $\theta$ versus $t$ curve is concave up, then $\alpha \gt 0$
If the $\theta$ versus $t$ curve is concave down, then $\alpha \lt 0$
On the graph, we can see that the $\theta$ versus $t$ curve is concave down.
Thus, $~~\alpha \lt 0$
That is, the angular acceleration $\alpha$ is negative.
