Which one of the following is employed as a tranquilizer drug?

Tranquilizers are neurologically active drugs used to treat stress, anxiety and mild or severe mental diseases; they relieve tension by inducing a sense of well-being.
Valium (diazepam) is a benzodiazepine tranquilizer; others are chlordiazepoxide, meprobamate, equanil and serotonin-related drugs.
Mifepristone is a synthetic steroid used as an antifertility ('morning after') drug.
Promethazine is an antihistamine, and naproxen is a non-steroidal analgesic (anti-inflammatory).
Hence the tranquilizer is valium.