Which of the following is the symbol for a $NAND$ gate?

The figure shows a combination of logic gates. To what single gate is this combination equivalent?

Input waveforms $A$ and $B$ as shown in Fig-$I$ are applied to the combination of gates as shown in Fig-$II$. Which of the waveforms shown in Fig-$(i)$ to Fig-$(iv)$ correctly represents the output waveform $Y$?

The Boolean equation of a $NOR$ gate is:

The following figure shows a logic gate circuit with two inputs $A$ and $B$ and the output $Y$. The voltage waveforms of $A$,$B$,and $Y$ are as given. The logic gate is:

The given circuit shows a combination of logic gates. Identify the equivalent gate.