What is the SI unit of electric field strength?

The SI unit of electric field strength is indeed expressed as Newton per Coulomb. Electric field strength describes the force experienced by a unit positive charge placed in an electric field. By definition, one Newton per Coulomb means that when a charge of one Coulomb is placed in the electric field, it experiences a force of one Newton.

This relationship is derived directly from the formula for electric field strength, which is defined as (E = \frac{F}{q}), where (E) is the electric field strength, (F) is the force acting on the charge, and (q) is the charge. When we express the units, force is measured in Newtons (N) and charge in Coulombs (C), leading to the unit of electric field strength being N/C or Newton per Coulomb.

Other options, while important in their respective contexts, do not represent electric field strength. Joule measures energy, Volt refers to electric potential, and Ampere is the unit of electric current. Each of these has distinct definitions and applications separate from that of electric field strength.