\documentclass[12pt,a4paper]{article}

\usepackage{tikz}
\usetikzlibrary{lindenmayersystems}

\usepackage{caption,subcaption}

\renewcommand{\subfigurename}{Figur}
\renewcommand{\thesubfigure}{\arabic{subfigure}}

\def\trianglewidth{2cm}%
\pgfdeclarelindenmayersystem{Sierpinski triangle}{
    \symbol{X}{\pgflsystemdrawforward}
    \symbol{Y}{\pgflsystemdrawforward}
    \rule{X -> X - Y + X + Y - X}
    \rule{Y -> YY}
}%

\newcommand{\Sierpinski}[1]{
\begin{tikzpicture}[l-system={step=\trianglewidth/(2^#1), order=#1, angle=-120}]
    \fill [black] (0,0) -- ++(0:\trianglewidth) -- ++(120:\trianglewidth) -- cycle;
    \draw [draw=none] (0,0) l-system
    [l-system={Sierpinski triangle, axiom=X},fill=white];
\end{tikzpicture}}

\begin{document}%

\begin{figure}[!htbp]
        \centering
        \foreach \level in {0,...,4}
        {%
        \pgfmathtruncatemacro{\levelName}{1+\level}
        \begin{subfigure}[b]{0.15\textwidth}
            \Sierpinski{\level}
            \caption{}
            \label{fig:I.9.\levelName}
        \end{subfigure}
        }
\end{figure}

\end{document}