at what time ABC or D is the writer accelerating upwardwhat is the magnitude of the maximum acceleration experienced by the rider

We will have the following:
From the graph we can see that the point at which it is accelerating upward is at point C. [We can see that where the slope of the function is positive the acceleration will be positive]
We determine the maximum magnitude of the acceleration of the rider by using the maximum force, that is:
[tex]F=m\cdot a\Rightarrow1.20N=a=\frac{1.20N}{m}[/tex]So, the maixmum magnitude of the acceleration will depend on the mass of the rider. Now, since the mass cannot be 0 kg since that would undetermine the expression we know then that the best approximation is:
[tex]a=\frac{1.20N}{m}[/tex]Where "m" is the mass of the rider in kg. Since we are not given a mass, then the acceleration can only be expressed as the force divided by the mass of the rider.