A semiconducting device is wired in a series circuit alongside a...

A $p$-$n$ junction diode allows current to flow when it is forward biased and offers very high resistance when it is reverse biased.

When the battery's polarity is reversed, the $p$-$n$ junction becomes reverse biased, causing the current to drop to nearly zero.

In contrast, $p$-type, $n$-type, and intrinsic semiconductors conduct current equally in both directions regardless of the battery's polarity.

Therefore, the device exhibiting this unidirectional conduction is a $p$-$n$ junction.