The given sentence is in active voice and needs to be converted to passive voice. The subject "somebody" in the active voice becomes the agent in the passive voice, and "was watching me" becomes "was being watched". Hence, the correct sentence is: "I didn't realise that I was being watched."
\begin{itemize}
\item Option (b) I didn't realise that I was being watching: This is incorrect because "being" should be followed by the past participle "watched" in passive voice.
\item Option (c) I didn't realised that I was being watched: "Realised" should be in the base form, "realise", because of the auxiliary verb "did".
\item Option (d) I didn't realise that I was been watched: "Been" is incorrect here. In passive voice, "being" should be used, not "been".
\end{itemize}