La documentazione prima di postare BISOGNA leggere la documentazione
infatti c’era l’esempio
ecco il codice modificato
`
\documentclass[preview=true]{standalone}
\usepackage{cmap}
\usepackage{fixltx2e}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[italian]{babel}
\usepackage{lmodern}
%\usepackage{textcomp}
%\usepackage{bbding}
%\usepackage{mparhack,relsize}
\usepackage{amsmath}
\usepackage{amssymb}
%\usepackage{arev}
\usepackage{isomath}
\usepackage{microtype}
\usepackage{geometry}
\geometry{a4paper,top=3cm,bottom=3cm,left=2cm,right=2cm}
\usepackage{tkz-euclide}
\usetkzobj{all}
\usepackage{circuitikz}
\usepackage{tkz-tab}
\usepackage{tkz-linknodes}
\usetikzlibrary{calc}
\usetikzlibrary{hobby}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usetikzlibrary{decorations}
\usetikzlibrary{shapes,arrows,trees,positioning,through,intersections}
\usetikzlibrary{shapes.misc,shapes.geometric,shapes.symbols,shapes.geometric}
\usetikzlibrary{shapes.gates.logic.US,shapes.gates.logic.IEC}
\usepackage{forest}
\tikzset{primo/.style={circle,draw,thick}, }
\begin{document}
\begin{forest}
[210
]
\node at (current bounding box.south)
[below=1ex,primo]
{A};
\end{forest}
\begin{forest}
[210
[21]
[10]
]
\node at (current bounding box.south)
[below=1ex,primo]
{B};
\end{forest}
\begin{forest}
[210
[21
[7,primo]
[3,primo]
]
[10]
]
\node at (current bounding box.south)
[below=1ex,primo]
{C};
\end{forest}
\begin{forest}
[210
[21
[7,primo]
[3,primo]
]
[10
[5,primo]
[2,primo]
]
]
\node at (current bounding box.south)
[below=1ex,primo]
{D};
\end{forest}
\end{document}
scusate
ps se c'è un'idea migliore…
`