The correct answer is D. Joule.
A volt is the unit of electric potential difference or electromotive force.
An erg is a unit of energy in the CGS system of units. It is equal to the work done by a force of one dyne acting through a distance of one centimeter.
An ohm is the unit of electrical resistance in the International System of Units (SI). It is defined as the resistance between two points on a conductor when a constant current of one ampere produces a potential difference of one volt between the points.
A joule is the SI unit of energy, work, or heat. It is equal to the work done by a force of one newton acting through a distance of one meter.
The MKS system is a system of units in which the metre, kilogram, and second are the base units. The joule is the unit of energy in this system.