Which one of the following circuits is **NOT** equivalent to a 2-input XNOR (exclusive NOR) gate?

Let the two inputs be $A$ and $B$. The output of a 2-input XNOR gate is:

Analyzing each option:

• (A) An XOR gate followed by a NOT gate produces $\overline{A \oplus B} = A \odot B$ (XNOR).

• (B) Both inputs inverted before XOR, followed by an inverter:

  $$\overline{\overline{A} \oplus \overline{B}} = \overline{A \oplus B} = A \odot B \quad (\text{XNOR})$$

• (C) One input inverted into an XOR gate produces $\overline{A} \oplus B = \overline{A\oplus B} = A \odot B$ (XNOR).

• (D) The circuit implements a different logic function that does not simplify to $A \odot B$.

Hence, circuit (D) is NOT equivalent to an XNOR gate.