Concept:
The word \textit{treacherous} comes from the noun \textit{treachery}, meaning betrayal or deceit. It is used to describe people, behaviour, or situations that cannot be trusted.
Explanation:
\textit{Treacherous} refers to someone or something that is likely to betray trust or cause harm in a deceptive way. It can describe both people (who betray others) and situations (that are dangerous or unstable).
For example:
\begin{itemize}
\item He proved to be a treacherous friend who revealed secrets.
\item The mountain roads become treacherous during heavy snowfall.
\end{itemize}
Option (A) is incorrect because loyal is the opposite of treacherous.
Option (B) refers to responsibility, which is unrelated.
Option (D) means true, which again is the opposite of betrayal.
Thus, option (C) best matches the meaning because something treacherous is not dependable or trustworthy.