An AC current flowing in a conductor will set up:

An alternating current (AC) flowing in a conductor establishes an alternating magnetic field around that conductor due to the principles of electromagnetism. When electric current flows through a wire, it generates a magnetic field that encircles the conductor.

With AC, the direction of the current changes periodically, which in turn causes the magnetic field to also shift direction in sync with the current. This relationship is described by Ampère's circuital law and is integral to the workings of transformers, inductors, and many electrical devices.

The specifics of this phenomenon are grounded in Faraday’s law of electromagnetic induction, highlighting the direct correlation between changing electric currents and the magnetic fields they produce. It is this alternating magnetic field that forms the basis for many applications in electrical engineering and technology.

The other options do not accurately capture the nature of AC current and its effects. An alternating current field or a periodically changing voltage tangent to the conductor would not correctly represent the influence of AC current in this context.