\resizebox{0.5\textwidth}{!}{
\begin{tikzpicture}[node distance=1.75cm]
\node[terminal={below}{$x[n]$}] (x) at (0,0) {};
\node[terminal={below}{}, right of=x] (00) {};
\node[terminal={below}{}, right of= 00] (01) {};
\node[terminal={below}{}, right of=01] (02) {};
\node[terminal={below}{$y[n]$}, right of=02] (y) {};

\coordinate[below of=00] (10) {};
\node[terminal={below}{}, below of=01] (11) {};
\coordinate[below of=02] (12) {};

%
\draw[zpath={right}] (11) to (01);

%
\draw[solid, \thickness] (00) to (10);
\draw[solid, \thickness] (02) to (12);
%

%
\draw[amark={$-a$}{above}] (00) to (01);
\draw[amark={$1$}{above}] (10) to (11);

%
\draw[amark={$1$}{above}] (01) to (02);
\draw[amark={$a$}{above}] (12) to (11);

\draw[amark] (x) to (00);
\draw[amark] (02) to (y);
\node[black, fill=black!10, below=1.5cm of y] {(A)}; 
\end{tikzpicture}
}

\resizebox{0.5\textwidth}{!}{
\begin{tikzpicture}[node distance=1.75cm]
\node[terminal={below}{$x[n]$}] (x) at (0,0) {};
\node[terminal={below}{}, right=1cm of x] (00) {};
\node[terminal={below}{}, right=1.25cm of 00] (01) {};
\node[terminal={below}{}, right=1cm of 01] (02) {};
\node[terminal={below}{}, right=1.25cm of 02] (03) {};
\node[terminal={below}{$y[n]$}, right=1cm of 03] (y) {};

\node[terminal={below}{}, below of=00] (10) {};
\coordinate[below of=01] (11) {};
\coordinate[below of=02] (12) {};
\node[terminal={below}{}, below of=03] (13) {};

%
\draw[zpath={left}] (00) to (10);
\draw[amark={$1$}{below}] (10) to (11);
\draw[solid, \thickness] (11) to (01);

%
\draw[zpath={right}] (03) to (13);
\draw[amark={$a$}{below}] (13) to (12);
\draw[solid, \thickness] (12) to (02);

%
\draw[amark={}{right}] (x) to (00);
\draw[amark={$-a$}{above}] (00) to (01);
\draw[amark={}{right}] (01) to (02);
\draw[amark={}{right}] (03) to (y);

\draw[amark={$1$}{above}] (02) to (03);


\node[black, fill=black!10, below=1.5cm of y] {(B)}; 
\end{tikzpicture}
}

\resizebox{0.5\textwidth}{!}{
	\begin{tikzpicture}[node distance=1.75cm]
	\node[terminal={below}{$x[n]$}] (x) at (0,0) {};
	\node[terminal={below}{}, right of=x] (00) {};
	\node[terminal={below}{}, right of= 00] (01) {};
	\node[terminal={below}{}, right of=01] (02) {};
	\node[terminal={below}{$y[n]$}, right of=02] (y) {};
	
	\coordinate[below of=00] (10) {};
	\node[terminal={below}{}, below of=01] (11) {};
	\coordinate[below of=02] (12) {};
	
	%
	\draw[amark={$a$}{right}] (11) to (01);
	
	%
	\draw[solid, \thickness] (00) to (10);
	\draw[solid, \thickness] (02) to (12);
	%
	
	%
	\draw[amark={$z^{-1}$}{above}] (00) to (01);
	\draw[amark={$-1$}{above}] (10) to (11);
	
	%
	\draw[amark={$1$}{above}] (01) to (02);
	\draw[amark={$z^{-1}$}{above}] (12) to (11);
	
	\draw[amark] (x) to (00);
	\draw[amark] (02) to (y);
	\node[black, fill=black!10, below=1.5cm of y] {(C)}; 
	\end{tikzpicture}
}

\resizebox{0.5\textwidth}{!}{
	\begin{tikzpicture}[node distance=1.75cm]
	\node[terminal={below}{$x[n]$}] (x) at (0,0) {};
	\node[terminal={below}{}, right of=x] (00) {};
	\node[terminal={below}{}, right of= 00] (01) {};
	\node[terminal={below}{}, right of=01] (02) {};
	\node[terminal={below}{$y[n]$}, right of=02] (y) {};
	
	\coordinate[below of=00] (10) {};
	\node[terminal={below}{}, below of=01] (11) {};
	\coordinate[below of=02] (12) {};
	
	%
	\draw[amark={$z^{-1}$}{right}] (01) to (11);
	
	%
	\draw[solid, \thickness] (00) to (10);
	\draw[solid, \thickness] (02) to (12);
	%
	
	%
	\draw[amark={$1$}{above}] (00) to (01);
	\draw[amark={$a$}{above}] (11) to (10);
	
	%
	\draw[amark={$-a$}{above}] (01) to (02);
	\draw[amark={$1$}{above}] (11) to (12);
	
	\draw[amark] (x) to (00);
	\draw[amark] (02) to (y);
	\node[black, fill=black!10, below=1.5cm of y] {(D)}; 
	\end{tikzpicture}
}