Step 1: Concept
This is the general formula for the linear operator $\frac{1}{D - a}$ acting on any function $X$.
Step 2: Meaning
The operation is equivalent to solving the linear differential equation $\frac{dy}{dx} - ay = X$.
Step 3: Analysis
Using the integrating factor $e^{\int -a dx} = e^{-ax}$, the solution is $y e^{-ax} = \int X e^{-ax} dx$.
Step 4: Conclusion
Multiplying both sides by $e^{ax}$ gives $y = e^{ax} \int X e^{-ax} dx$, which is the identity for the operator.
Final Answer: (A)