\documentclass{article}
\usepackage{tikz}
\usepackage{xepersian}
\begin{document}
\begin{tikzpicture}
\filldraw [black]
(2,1) circle (2pt)
(1.65,1.65) circle (2pt)
(1,2) circle (2pt)
(0.25,1.5) circle (2pt);
\draw (1.65,1.65)-- (2,1);
\draw (1,2)-- (1.65,1.65);
\draw (1.65,1.65)-- (0.25,1.5);
\draw (0.25,1.5)--(1,2);
\node[right] at(2,1) {$d$};
\node[right] at(1.65,1.65) {$c$};
\node[above] at(1,2) {$b$};
\node[left] at(0.25,1.5) {$a$};
\end{tikzpicture}


\end{document}