As velocities are tangent, the value of both Particle A and Particle B would be same for that point O (Intersecting point)
a = v / t
Here, v = 7, t = 6
So, a = 7/6
a = 1.17
As the graph is decreasing, value of acceleration would be negative.
So, a = -1.17 m/s²
In short, Your Answer would be Option C
Hope this helps!