What are the invisible lines that map out the magnetic field around a magnet called?

The correct answer identifies the invisible lines that illustrate the magnetic field around a magnet as magnetic field lines. These lines represent the direction and strength of the magnetic field, radiating from the north pole of a magnet and circling back to the south pole. The density of these lines in a given area indicates the strength of the magnetic field: the closer the lines are to each other, the stronger the magnetic field in that region.

Magnetic field lines provide a visual and conceptual tool that helps in understanding how magnetic forces work in space around a magnet. They are crucial for various applications in physics and engineering, as they facilitate the analysis of magnetic interactions and behaviors.

The other options refer to concepts that are related but do not specifically denote the mapping of a magnetic field. Magnetic flux, for example, refers to the quantity of magnetic field passing through a given area, which is related but distinct from the visual representation provided by magnetic field lines. Magnetic eras and magnetic currents do not accurately describe the phenomenon of mapping a magnetic field.