Use the following rule set. Slater's rules: write the configuration in groups $(1s)(2s,2p)(3s,3p)(3d)(4s,4p)(4d)(4f)(5s,5p)\dots$ For an electron in an $ns$ or $np$ group, every other electron in the same group contributes $0.35$ ($0.30$ for the $1s$ group), every electron in the $(n-1)$ shell contributes $0.85$, every electron in the $(n-2)$ or lower shells contributes $1.00$, and electrons in groups to the right contribute $0$. For an electron in an $nd$ or $nf$ group, every other electron in the same group contributes $0.35$, every electron in all groups to its left contributes $1.00$, and groups to the right contribute $0$. $Z^{\ast }=Z-S$. Iron ($Z=26$) is $(1s{)}^2(2s,2p{)}^8(3s,3p{)}^8(3d{)}^6(4s{)}^2$. The effective nuclear charge on a $3d$ electron of iron is:

1. A5.55
2. B8.95
3. C6.25
4. D7.45