Answer
The $a$ axis should intersect the $t$ axis between the points $B$ and $C$
Work Step by Step
When $x = 0,$ then $a = 0$
If $x \lt 0$ and the object is moving in the negative direction, then the acceleration is in the positive direction.
When $x = -x_m$, then $a$ is at its most positive value.
Therefore, the $a$ axis should intersect the $t$ axis between the points $B$ and $C$
