Out of the given options, the non-ohmic device is option 3, which is a semiconductor diode.

An ohmic device is one that follows Ohm`s law, which states that the current flowing through a conductor is directly proportional to the voltage applied across it. In other words, the resistance of an ohmic device remains constant regardless of the applied voltage.

Option 1, which is a conducting copper coil, is an example of an ohmic device as the resistance remains constant.

Option 2, an electric heating coil, may not strictly follow Ohm`s law. As the temperature increases, the resistance of the coil may also change due to the change in its physical properties. However, it can be considered approximately ohmic.

Option 3, the semiconductor diode, is a non-ohmic device. It does not follow Ohm`s law because its resistance is not constant but depends on the direction of current flow. It allows current to flow easily in one direction (forward bias) but restricts or blocks the current flow in the opposite direction (reverse bias).

Option 4, the rheostat, is an adjustable resistor that can vary the resistance. While the resistance can be changed, it can still be considered an ohmic device as