Answer:
C. 1.00 m/s^2
Step-by-step explanation:
We can solve the problem by using Newton's second law:

where
F is the net force acting on an object
m is the mass of the object
a is its acceleration
In this case, the net force applied by Amber is F=35 N, while the total mass of the brother+the wagon is

Therefore, we can re-arrange the previous equation to find the acceleration:
