documentclass{article}
usepackage{tikz}
usetikzlibrary{positioning}

begin{document}
    begin{tikzpicture}

      node [draw] (a) {a};
      node [draw, right=of a] (b) {b};
      node [draw, right=of b] (c) {c};
      node [draw, below=of c] (f) {f};

      draw (a) to[out=0,in=180] (f);

    end{tikzpicture}
end{document}

documentclass{article}
usepackage{tikz}
usetikzlibrary{positioning}

begin{document}
    begin{tikzpicture}

      node [draw] (a) {a};
      node [draw, right=of a] (b) {b};
      node [draw, right=of b] (c) {c};
      node [draw, below=of c] (f) {f};

      draw (a) ..controls (b) and ({a |- f}).. (f);

    end{tikzpicture}
end{document}