Step 1: Concept
For hyperbolic functions like $\cosh ax$, we substitute $D^{2} = a^{2}$ in the operator.
Step 2: Meaning
$P.I. = \frac{1}{D^{2} + 1} \cosh 3x$. Here $a = 3$, so $a^{2} = 9$.
Step 3: Analysis
Substituting $D^{2} = 9$ into the denominator: $\frac{1}{9 + 1} \cosh 3x = \frac{1}{10} \cosh 3x$.
Step 4: Conclusion
Since the denominator is non-zero, this direct substitution yields the particular integral.
Final Answer: (A)