May be a solution, but why all the text is not in the left column and the single figure in the right column?
\setuppapersize[A4] \setuplayout[ backspace=15mm, width=middle, topspace=15mm, height=middle, header=0pt, footer=0pt, ] \mainlanguage[fr]
\usemodule[tikz] \usetikzlibrary[calc] \starttext \blank[small] \leftaligned{\bf Partie A} \blank[small] \startitemize[n][stopper=),style=bold] \item \input knuth \item \input knuth \stopitemize \startcolumns \blank[small] \leftaligned{\bf Partie B} \blank[small] \startitemize[n][stopper=),style=bold] \item \item \item \startitemize[a][stopper=),style=bold] \item \input knuth \item \stopitemize \stopitemize \column \starttikzpicture[level 1/.style={level distance=30mm,sibling distance=40mm},level 2/.style={level distance=25mm,sibling distance=25mm},grow=right] \node{} child{node[blue]{$B$} child{node[blue]{$B$} edge from parent node[below=0.2cm]{$0,75$}} child{node[red]{$R$} edge from parent node[above=0.2cm]{$0,25$}} edge from parent node[below=0.2cm]{$0,75$}} child{node[red]{$R$} child{node[blue]{$B$} edge from parent node[below=0.2cm]{$0,75$}} child{node[red]{$R$} edge from parent node[above=0.2cm]{$0,25$}} edge from parent node[above=0.2cm]{$0,25$}}; \stoptikzpicture \stopcolumns \stoptext