Which of the following is a non-barbiturate?

The correct choice identifies Carisoprodol, commonly known as Soma, as a non-barbiturate. This substance is classified as a muscle relaxant, primarily used for the relief of musculoskeletal pain. Unlike barbiturates, which are a specific class of sedative medications that act on the central nervous system, Carisoprodol has a different mechanism of action and is utilized mostly for its muscle-relaxing properties rather than as a sedative or anxiolytic.

In contrast, the other substances listed have classifications that align them with barbiturates or similar sedative-hypnotics. Chloral Hydrate and Methaqualone are both recognized as sedative-hypnotics and would be associated with the effects typically observed with barbiturates. Diazepam, although a benzodiazepine, also serves similar functions as a sedative and anxiolytic, but is not considered a barbiturate. This context reinforces Carisoprodol's unique classification as a non-barbiturate in this question.