In Boger Boenninghausen's Characteristics and Repertory (BBCR), the rubric "Opisthotonus" refers to a severe form of muscle spasm where the back is arched, and the head and heels are bent backward. This is a general physical symptom indicating a significant neurological involvement or a systemic reaction. Therefore, it is listed under the chapter "**Sensation and Complaints in general**" in the BBCR, which encompasses general pathological states, sensations, and involuntary movements affecting the body as a whole.
The other options are less likely:
\begin{itemize}
\item Mind deals with mental and emotional symptoms.
\item Fever focuses on symptoms associated with febrile conditions.
\item Lower extremities pertains to symptoms localized in the legs and feet.
\end{itemize}
Thus, the general symptom of opisthotonus is appropriately located under the "**Sensation and Complaints in general**" chapter in Boger Boenninghausen's Repertory.