The circuit consists of four NAND gates.
The first two NAND gates have their inputs tied together, acting as NOT gates. Their outputs are X and Y.
These outputs are fed into the third NAND gate. The output of this gate is:
y′=X⋅Y
Using De Morgan's theorem:
y′=X+Y=X+Y
This output is fed into the fourth NAND gate, which also has its inputs tied together, acting as a NOT gate. The final output is:
Output =X+Y
This represents the Boolean expression for a NOR gate.
Answer: NOR gate