Which one of the following expressions does NOT represent exclusive NOR of $x$ and $y$?

The exclusive NOR (XNOR) operation is defined as:

Let's evaluate each option:

• (A) $xy + x^{\prime}y^{\prime}$ is the canonical sum-of-products form of XNOR.

• (B) $x \oplus y^{\prime} = x (y^{\prime})' + x^{\prime} y^{\prime} = xy + x^{\prime}y^{\prime} = x \odot y$ (XNOR).

• (C) $x^{\prime} \oplus y = x^{\prime} y^{\prime} + (x^{\prime})' y = x^{\prime}y^{\prime} + xy = x \odot y$ (XNOR).

• (D) $x^{\prime} \oplus y^{\prime} = x^{\prime} (y^{\prime})' + (x^{\prime})' y^{\prime} = x^{\prime}y + xy^{\prime} = x \oplus y$ (XOR, not XNOR).

Therefore, expression (D) represents XOR, not XNOR.