Step 1: Meaning of the term.
The term "Indelible" refers to something that cannot be erased, removed, or forgotten. It is used for permanent impressions or marks.
Step 2: Analysis of the options.
\begin{itemize}
\item Inaudible refers to something that cannot be heard.
\item Irritable refers to a person easily angered.
\item Unsuitable refers to something that is not appropriate.
\item Indelible means something that cannot be erased or removed, fitting the description of the question.
\end{itemize}
Step 3: Conclusion.
Thus, the correct answer is (D) Indelible.
Final Answer: \[ \boxed{\text{Indelible is the correct answer.}} \]