Step 1: Solving the second-order linear differential equation.
The solution to the differential equation is found by solving the homogeneous part and particular solution separately. The final solution is \( y = (c_1 + c_2x) e^x + \frac{e^{3x}}{8} \).
Step 2: Conclusion.
Thus, the correct answer is option (C).
Final Answer:
\[
\boxed{\text{(C) } y = (c_1 + c_2x) e^x + \frac{e^{3x}}{8}}
\]