it is not clear, which table you like to obtain. if the first one, and if the table on the end will longer than one page, than solution can be:
\documentclass{article}
\usepackage{geometry}
\usepackage[table]{xcolor}
\usepackage{longtable, tabularx}
%-------------------------------- show page layout, only for test
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%
\begin{document}
with \verb+longtable+:
\begin{longtable}{|*{2}{p{\dimexpr0.5\linewidth-2\tabcolsep-\arrayrulewidth\relax}|}}
\hline
\multicolumn{2}{| c |}{\cellcolor{gray!25} Begin of Table}\\
\hline
A) Something & B) something j \\
\hline
C) Something & D) something j\\
\hline
\end{longtable}
with \verb+tabularx+:
\begin{center}
\begin{tabularx}{\linewidth}{|X|c|X|c|}
\hline
\rowcolor{gray!25}
\multicolumn{4}{| l |}{What is capital of Canada?} \\
\hline
Berlin & C & Ottawa & A \\
\hline
Rome & D & Canberra & B \\
\hline
\end{tabularx}
\end{center}
\end{document}
giving

longtable
? is many page long? from table image is seems thattabular*
ortabularx
can be is sufficient.\multicolumn{2}{..}{...}
directive. Please make the code compilable -- so that it actually generates the error you're looking to fix.