Which of the following is a buffer [MP PMT 1991; BHU 1995]

• A.                 \[NaOH+C{{H}_{3}}COONa\]
• B.                 \[NaOH+N{{a}_{2}}S{{O}_{4}}\]
• C.                 \[1\times {{10}^{-4}}\]
• D.                 \[N{{H}_{4}}OH+C{{H}_{3}}COON{{H}_{4}}\]