The factor that affects how easily an electron can be removed from an atom is the
a. structure of the atom.
b. size of the atom.
c. temperature of the environment.
d. surrounding atoms.

Temperature of the environment, surrounding atoms and size of atom are all factors which affect removal of electron. Increase in temperature increases the probability of removal of electron. But size of the atom plays very important role in removal of electrons. In case of large sized atom, the nuclear attraction of nucleus on valence electrons decreases which eases the removal of electrons. Hope this helps.