Correct Answer: C
Explanation:
The sulphur trioxide electrophile arises in one of two ways depending on which sort of acid you are using.
Concentrated sulphuric acid contains traces of SO3 due to slight dissociation of the acid.
Fuming sulphuric acid, H2S2O7, can be thought of as a solution of SO3 in sulphuric acid - and so is a much richer source of the SO3.
Sulphur trioxide is an electrophile because it is a highly polar molecule with a fair amount of positive charge on the sulphur atom. It is this which is attracted to the ring electrons