= Solution
To identify the unlabelled individuals in the original <pedigree>, call the three children of $A,B$ from left to right $U,V,W$. Then $G$ has parents $C,U$, $D$ has parents $V,W$, and $E,F$ have parents $G,D$. The three <full siblings> $U,V,W$ are noninbred because $A,B$ are unrelated noninbred <founders in a pedigree>. Their pairwise <kinship coefficients> are
$$
\phi_{UV}=\phi_{UW}=\phi_{VW}=\frac{\phi_{AA}+\phi_{BB}}4=\frac14.
$$
Since $D$ is the child of the sibling pair $V,W$, its <inbreeding coefficient> is
$$
\boxed{F_D=\phi_{VW}=\frac14.}
$$
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