A

Step 1: Identify the expression for potential energy: U = ax^2 - bx.
Step 2: Calculate the force acting on the system, which is the negative gradient of potential energy: F = -\frac{dU}{dx} = -\frac{d}{dx}(ax^2 - bx) = -2ax + b.
Step 3: Set the force equal to zero for equilibrium: -2ax + b = 0, which gives us x = \frac{b}{2a}.
Thus, the net force acting on the system is correctly identified as [A] \frac{b}{2a}.
Step 4: To find the equilibrium position, equate the first derivative of potential energy to zero. The expression derived indicates that at this value of x, the force is balanced.
Therefore, A.