Please help me understand the explaination as well as the answer

The activation energy to remove an iron atom from the $FCC$ lattice is $1\mathrm{.}7eV.$ At $800C,$ how many

vacancies are there per unit cell?

A$.1\mathrm{.}0-3\mathrm{.}0({10}^{-8})$ vacancies

B$.3\mathrm{.}0-5\mathrm{.}0({10}^{-8})$ vacancies

C$.3\mathrm{.}0-5\mathrm{.}0({10}^{-11})$ vacancies

D$.5\mathrm{.}0-7\mathrm{.}0({10}^{-11})$ vacancies