Step 1: Understanding the order of a differential equation.
The order of a differential equation is determined by the highest derivative of the unknown function in the equation.
Step 2: Identifying the highest derivative.
In the given differential equation:
\[
\frac{d^2y}{dx^2} - 2\frac{dy}{dx} + 3y = 0
\]
The highest derivative is \( \frac{d^2y}{dx^2} \), which is the second derivative of \( y \).
Step 3: Conclusion.
Therefore, the order of the differential equation is 2, corresponding to option (B).