Explanation:

Surface Current and Magnetic Vector Potential:
When dealing with a surface current, the magnetic vector potential (A) is used to represent the magnetic field generated by the current. The magnetic vector potential is defined as the vector field whose curl gives the magnetic field.

Current Element for Surface Current:
For a surface current, the current element used in the expression for the magnetic vector potential is the surface current density (K) multiplied by the differential area element (dS). This current element is used to calculate the magnetic vector potential at a point in space due to the surface current.

Correct Answer:
The current element of the magnetic vector potential for a surface current will be K dS. This means that the correct option is option 'C' - K dS. This expression represents the contribution of the surface current to the magnetic vector potential at a specific point.

Explanation:
Option 'C' - K dS is the correct answer because it correctly represents the current element for a surface current when calculating the magnetic vector potential. When dealing with surface currents, it is important to use the correct current element in the expression for the magnetic vector potential to accurately calculate the magnetic field generated by the current.
By understanding the relationship between surface currents and the magnetic vector potential, we can effectively analyze and predict the magnetic field generated by surface currents in various electrical engineering applications.