Step 1: Check each scenario.
(a) \(T\) opposite \(O\): place \(O\) on one wall, \(T\) facing it on the other—no same-wall adjacency involved—\textit{possible}.
(b) \(T,H,E\) on one wall: arrange as \(H\text{–}E\text{–}T\) (or any order avoiding a common word); the other wall gets \(M,O,R\) with \(O\) between \(M,R\)—\textit{possible}.
(d) \(M\) opposite \(O\): trivially \textit{possible}.
Step 2: Why (c) is impossible.
If \(\{H,M,R\}\) occupy one wall, the other wall must be \(\{E,O,T\}\).
On a 3-slot wall, the letter in the middle is adjacent to \emph{both} ends; whichever position \(O\) takes, it becomes adjacent to at least one of \(E\) or \(T\), violating both bans simultaneously (centre: adjacent to both; end: adjacent to the middle which must be \(E\) or \(T\)).
Hence such a distribution cannot satisfy the constraints.
\[
\boxed{\text{(c)}}
\]