The correct option is C [Ar]3d5
Iron is a transition element (d-block). Its atomic number is 26 i.e. 26 electrons.
Its electronic configuration is,
Fe−1s2 2s2 2p6 3s2 3p6 4s2 3d6 or
[Ar]4s2 3d6.
Fe3+ is formed by loosing two electron from 4s and one electron from 3d.
So, the electronic configuration of Fe3+ will be [Ar]3d5.
Hence, option (c) is the correct answer.