Step 1: Concept
The complementary function (C.F.) is the general solution of the homogeneous part $y'' - 3y' + 2y = 0$.
Step 2: Meaning
The auxiliary equation is $m^{2} - 3m + 2 = 0$.
Step 3: Analysis
Factoring the equation: $(m - 2)(m - 1) = 0$, which gives roots $m_{1} = 2$ and $m_{2} = 1$.
Step 4: Conclusion
For distinct real roots, $C.F. = c_{1}e^{m_{1}x} + c_{2}e^{m_{2}x} = c_{1}e^{2x} + c_{2}e^{x}$.
Final Answer: (D)