Step 1: Concept
For a linear differential equation $\frac{dy}{dx} + Py = Q$, the Integrating Factor (I.F.) is $e^{\int P dx}$.
Step 2: Meaning
Here, $P = -2$.
Step 3: Analysis
I.F. $= e^{\int -2 dx} = e^{-2x}$.
Step 4: Conclusion
The integrating factor is $e^{-2x}$.
Final Answer: (E)