Option a) Volt : is a unit of electrostatic potential.
Option b) Joule / Coulomb: Energy per unit charge. Electrostatic potential is electrostatic potential energy gained or lost in moving a unit charge in an electric field. Hence, Joule / Coulomb is also a unit of Electrostatic potential.
Option c) Newton/Coulomb : Force per unit charge which is electric field. Hence, option c) is not a unit of electrostatic potential.
Option d) Newton-metre/Coulomb: Newton-metre is Joule. Hence Joule / Coulomb is a unit of electrostatic potential.