(D) To find the number of unpaired electrons,we fill the $d$-orbitals according to Hund's rule:$3d^6$: $5$ orbitals,$4$ unpaired electrons.$3d^7$: $5$

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(D) To find the number of unpaired electrons,we fill the $d$-orbitals according to Hund's rule:$3d^6$: $5$ orbitals,$4$ unpaired electrons.$3d^7$: $5$ orbitals,$3$ unpaired electrons.$3d^8$: $5$ orbitals,$2$ unpaired electrons.$3d^9$: $5$ orbitals,$1$ unpaired electron.Thus,$3d^9$ has the smallest number of unpaired electrons $(1)$.