Which gate is used in digital circuits to perform universal logic operations?

Given $A$ and $B$ are input terminals. Logic $1 = > 5 \ V$,Logic $0 = < 1 \ V$. Which logic gate operation does the above circuit perform?

The logic circuit in the figure is equivalent to

In the given circuit as shown,the two input waveforms $A$ and $B$ are applied simultaneously. The resultant waveform $Y$ is:

Identify the logic gates represented by the given circuits $(a)$ and $(b)$ and write their truth tables.

$A$ $NOR$ gate and a $NAND$ gate are connected as shown in the figure. Two different sets of inputs are given to this setup. In the first case, the inputs to the gates are $A=0, B=0, C=0$. In the second case, the inputs are $A=1, B=0, C=1$. The output $D$ in the first case and second case respectively are