Since 2005, I have generally used this model to create multiple-choice tests for my students. I kindly ask you if there are packages or macros, or particular strategies to have squares with the letters A, B, C, D inside. I created the letter E with the underline (tabular) if, for pure typing error or distraction, I had not put the correct answer.
As you can see with the red arrows I do not have perfect squares and I think that all this depends on the letter used. But is it possible to find a solution to not use tabular? There is also another problem. Should I use aligned or other strategies to get a perfect alignment in mathematical mode? In which way could I have an optimal result?

Here there is my MWE:
\documentclass[leqno,11pt,landscape,a3paper]{extarticle}
\usepackage{graphicx,multicol,mathtools}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\begin{document}
\begin{multicols}{1}
\begin{enumerate}
\item La velocità di 1,00$\cdot 10^2$ m/s espressa in km/h è:
$\fbox{A}$ 36 km/h.\\
$\fbox{B}$ 360 km/h.\\
$\fbox{C}$ 27,8 km/h.\\
$\fbox{D}$ $3,\!60\cdot 10^{8}$ km/h.\\
$\fbox{E}$ \begin{tabular}{lllllllllllll}
\multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{6}{c}{} \\
\hline
\end{tabular}
\item In fisica nucleare si usa l'angstrom (simbolo: 1\AA=$10^{-10}$ m) e il fermi o femtometro (1 fm= $10^{-15}$ m). Qual è la relazione tra queste due unità di misura?
$\fbox{A}$ 1 \AA = $10^5$ fm.\\
$\fbox{B}$ 1 \AA = $10^{-5}$ fm.\\
$\fbox{C}$ 1 \AA = $10^{-15}$ fm.\\
$\fbox{D}$ 1 \AA = $10^{3}$ fm.\\
$\fbox{E}$ \begin{tabular}{lllllllllllll}
\multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{6}{c}{} \\
\hline
\end{tabular}
\end{enumerate}
\end{multicols}
\clearpage
\end{document}



Underfull \hbox (badness 10000) in paragraph at lines 54--55 Underfull \hbox (badness 10000) in paragraph at lines 54--55is warning you about the misuse of\\– David Carlisle Nov 25 '18 at 20:20\\at the end of a paragraph, simply delete the\\– David Carlisle Nov 25 '18 at 20:25alterqcmthat I use to do MCQs, its documentation is in French. https://www.ctan.org/pkg/alterqcm – AndréC Nov 25 '18 at 20:58alterqcmand a minimum of explanations. – AndréC Nov 25 '18 at 21:48