25
Other than verbatim I wouldn't know.
Verbatim environment:
\begin{verbatim}
word_a_a_a_a_a_b_c_dd
\end{verbatim}
Inline:
\verb|word_a_a_a_a_a_b_c_dd|
I couldn't get the underscore package to work, so I used the url package:
\usepackage{url}
\urlstyle{sf} % or rm, depending on your font
...
Foo \url{word_a_a_a_a_a_b_c_dd} bar.