https://tex.stackexchange.com/q/551501/35864

mathparse.tex

% based on https://tex.stackexchange.com/a/134836/35864
% by percusse (https://tex.stackexchange.com/users/3235/percusse)
\documentclass[tikz]{standalone}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}
\def\radi{{"\mathtt{a}","\mathtt{b}","\mathtt{c}","\mathtt{d}"}}
\foreach \x/\a [
    count=\n,
    evaluate=\x as \myvar using ({\radi[\x]})
    ] in {0/15,1/345,2/345,3/345}
    \draw[very thin, draw=red!50!white] 
        (0,0) circle [radius={\n}] 
        node at (\a:\n) {$\myvar$};


\end{tikzpicture}
\end{document}