$Oleum$ (also known as $Sulphan$) is

• A
  a mixture of $SO_{3}$ and $H_{2}SO_{3}$
• B
  $100\%$ conc. $H_{2}SO_{4}$
• C
  a mixture of gypsum and conc. $H_{2}SO_{4}$
• D
  $100\%$ oleum (a mixture of $SO_{3}$ in $H_{2}SO_{4}$)