Which of the following represents the $NAND$ gate?

In the given circuit,find the value of $Y$.

The following figure shows an $AND$ logic gate circuit with two inputs $A$ and $B$ and the output $C$. The voltage waveforms of $C$ will be:

The truth table given below is for which gate?
$A: 0, 0, 1, 1$
$B: 0, 1, 0, 1$
$C: 1, 1, 1, 0$

The Boolean equation $A + \bar{A} = .......$

Consider two $npn$ transistors as shown in the figure. If $0 \, V$ corresponds to false and $5 \, V$ corresponds to true,then the output at $C$ corresponds to: