Step 1: Concept
Recognize the expression as the derivative of a quotient: $d(\frac{x}{y}) = \frac{ydx - xdy}{y^2}$.
Step 2: Meaning
The equation is $d(\frac{x}{y}) = 0$.
Step 3: Analysis
Integrating both sides gives $\frac{x}{y} = c$, where $c$ is a constant.
Step 4: Conclusion
$x = cy$ or $y = mx$ (where $m=1/c$) is the equation of a straight line passing through the origin.
Final Answer: (A)