The group which leaves from the substrate in a nucleophilic substitution reaction is called?

Explanation

The group that leaves from the substrate in a nucleophilic substitution reaction is called the leaving group. In a nucleophilic substitution reaction, a nucleophile attacks the substrate molecule, resulting in the displacement of the leaving group. The leaving group is typically an atom or a functional group that can depart with its pair of electrons, forming a new bond with another atom or molecule. The departure of the leaving group creates a positive charge or a partial positive charge on the substrate, which facilitates the nucleophilic attack. Therefore, option a is correct.