Step 1: Concept
The second derivative $\frac{d^2y}{dx^2} = 0$ indicates that the rate of change of the slope is zero, meaning the relation between $x$ and $y$ is linear.
Step 2: Meaning
Integrating the condition $\frac{d^2y}{dx^2} = 0$ twice with respect to $x$ provides the functional form of the relationship.
Step 3: Analysis
First integration: $\frac{dy}{dx} = m$ (where $m$ is a constant slope). Second integration: $y = mx + c$ (where $c$ is a constant).
Step 4: Conclusion
Among the given options, $y = ax + b$ is the only expression that represents the standard general equation of a straight line.
Final Answer: (C)