答案1
\documentclass{article}
\usepackage{tikz, lmodern}
\usetikzlibrary{calc,tikzmark}
\begin{document}
\begin{tikzpicture}[overlay,remember picture]
\node [yshift=.6ex,rotate=45] at ( $(pic cs:A) !.5! (pic cs:B)$ ){ \fontsize{8}
{6}\selectfont\textbf{\color{black!40}RESTRICTED} };
\end{tikzpicture}
\begin{tabular}{llll}
1 & 2 & 3 & \tikzmark{B}4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
\tikzmark{A}1 & 2 & 3 & 4
\end{tabular}
\end{document}