\item \BDDToDNF

\begin{center}
\begin{forest}
for tree={circle, draw, no edge,
            minimum size=2em,
            inner sep=0pt,
            s sep=2mm,
            l sep=1mm}
[$f$, rectangle, draw,  tikz={\draw [line] () to (a.north);}
    [$a$, name=a, tikz={\draw [line] () to (b.north east); \draw [fulldot=.5] () to (d.north);}
        [$b$, name=b, tikz={\draw [fulldot=.5, bend left] () to (c.north east); \draw [line, bend right] () to (c.north west);}
            [$c$, name=c, s sep=20mm, tikz={\draw [line] () to (250:4); \draw [fulldot=.5] () to (d.north west);}
                [,phantom]
                [$d$, name=d, s sep=10mm, tikz={\draw [fulldot=.5] () to (287:5); \draw [line] () to (275:4.8);}]
            ]
        ]
        [,phantom]
    ]
]
\end{forest}
\end{center}