Step 1: Concept
The order of a differential equation is equal to the number of independent arbitrary constants in the general solution.
Step 2: Analysis
The expression is $y = (c_1 + c_2)\sin(x + c_3) - c_4e^x$.
Combine $(c_1 + c_2)$ into a single constant $K$. So, $y = K\sin(x + c_3) - c_4e^x$.
Step 3: Conclusion
The independent constants are $K, c_3,$ and $c_4$. Since there are 3 independent constants, the order of the differential equation is 3.
Final Answer: (C)