Oleum or fuming ${\mathrm{H}}_2{\mathrm{SO}}_4$ is

Sulphuric acid saturated with sulphur trioxide, i.e., ${\mathrm{H}}_2{\mathrm{ }\mathrm{S}}_2{\mathrm{O}}_7$

A mixture of sulphuric acid and nitric acid

Sulphuric acid which gives fumes of sulphur dioxide

A mixture of conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ and oil