\documentclass[12pt,a4paper]{book}
\usepackage{amsmath,amssymb,amsthm,paralist}
\usepackage{xepersian}

\begin{document}

\begin{itemize}\setlength{\itemsep}{0.1cm}
\item[1.]
\begin{inparaenum} \setlength{\plitemsep}{1cm}
\item[(الف)]$2i$ 
\item[(ب)]$-i$ 
\item[(پ)]$\frac{1}{2}-\frac{1}{2}i$ 
\item[(ث)]$18+i$ 
\item[(ج)]$-\frac{1}{5}+\frac{3}{5}i$ 
\item[(چ)]$1+i$ 
\item[(ح)]$0$ 
\end{inparaenum}
\item[2.]
\end{itemize}

\end{document}