The correct option is C 9.033×1020
1 mole of [Co(H2O)4Cl2]Cl formula unit contains 4 moles of O-atoms.
So, 38 millimoles of [Co(H2O)4Cl2]Cl contain 38×4=32 millimoles of O-atoms.
Again, 1 mole of O-atoms =6.022×1023 O-atoms.
Therefore, 32 millimoles of O-atoms =32×10−3×6.022×1023 O-atoms
=9.033×1020 O-atoms