documentclass[xcolor=table, t]{beamer}

usepackage{booktabs}
usepackage{tikz}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}

title{my presentation}
author{name}

begin{document}

begin{frame}
maketitle
end{frame}

begin{frame}
frametitle{title}

begin{table}[]%footnotesize
centering
medskip
begin{tabular}{lr}
toprule
Transizione & n. \
midrule
Ading{221}Bding{221}Cding{221}A & 54 \
Dding{221}Cding{221}Bding{221}Cding{221}D & 30 \
Dding{221}Cding{221}Bding{221}C & 23 \
bottomrule
end{tabular}
end{table}
end{frame}

end{document}