What is fuming sulphuric acid ?

Fuming sulphuric acid is

Benzene treated with fuming sulphuric acid.

Aniline on heating with fuming sulphuric acid gives

Benzene reacts with fuming sulphuric acid to give

When aniline is treated with fuming sulphuric acid at 475 K, it gives

When aniline is treated with fuming sulphuric acid at 475 K, it gives

Chloro benzene is sulphonated with fuming sulphuric acid. The major product formed is

Arrange the following in increasing order of reactivity towards sulphonation with fuming sulphuric acid. Benzene, toluene, methoxy benzene, chlorobenzene.

Arrange the following in increasing order of reactivity towards sulphonation with fuming sulphuric acid. Benzene, toluene, methoxy benzene, chlorobenzene.