The question asks to identify the property responsible for producing magnetic flux in a magnetic circuit, driven by electric current. Let's analyze the options:

Magnetomotive Force: The Flux Producer

Magnetomotive force (MMF) is the property that drives magnetic flux through a magnetic circuit. It is analogous to electromotive force (EMF) in electric circuits, which drives electric current. MMF is generated by electric currents flowing through coils of wire.

The formula for MMF is:

Where:

• $N$ represents the number of turns in the coil.
• $I$ represents the electric current flowing through the coil.

The unit of MMF is Ampere-turns (At). A higher MMF value generally leads to a greater magnetic flux in the circuit, assuming other factors remain constant. Therefore, MMF is precisely the "flux-producing ability of an electric current in a magnetic circuit."

Why Other Options Are Incorrect

• Electromotive force (EMF): EMF ($\mathcal{E}$) is the voltage or potential difference that drives electric current in an electric circuit. It is not directly responsible for producing magnetic flux in a magnetic circuit.
• Flux density ($B$): Flux density measures the amount of magnetic flux per unit area. It is the result or consequence of the MMF acting on the magnetic circuit, not the cause. It is defined as $B = \frac{\Phi}{A}$, where $\Phi$ is the magnetic flux and $A$ is the area.
• Magnetic field strength ($H$): Magnetic field strength ($H$) represents the MMF per unit length of the magnetic circuit. It indicates how strong the magnetic field is at a point in space and is related to MMF by $H = \frac{\text{MMF}}{l}$, where $l$ is the length of the magnetic circuit. While related, it's not the fundamental 'flux-producing ability' itself, but rather the force per unit length that causes the flux.

Based on these definitions, Magnetomotive Force is the correct term for the flux-producing ability in a magnetic circuit.