\documentclass[border=2pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{trees}
\begin{document}
\begin{tikzpicture}[level distance=1.2cm, sibling distance=1.5cm, edge from parent path={(\tikzparentnode) -- (\tikzchildnode)}]
  \node[circle, draw] {root}
    child { node[circle, draw] {A} }
    child { node[circle, draw] {B}
      child { node[circle, draw] {C} }
      child { node[circle, draw] {D} }
    };
\end{tikzpicture}
\end{document}
