Which atom has the largest ionization energy? multiple choice?

Ionization energy is the energy needed to remove one moles of electrons from one mole of gaseous atoms to form one mole of gaseous singly-charged cations, i.e.
X(g) → X⁺(g) + e⁻ …. ΔH = ionization energy

Ge, As, Se and Br are all Period 4 elements with atomic numbers 32, 33, 34 and 35 respectively. They have the same electronic configuration of the inner shells, and thus experience almost the same screening effect.

Among the 4 elements, Br has the largest ionization energy. This is because Br atom has the greatest nuclear charge (due to greatest atomic number) and the shortest distance between the nucleus and the electrons of the outermost shell. Therefore, the electrons in the outermost shell are attracted by the nucleus with the greatest attractive force, and thus the greatest energy is needed to remove an electron from the outermost shell.

You can look these up for yourself which I suspect is the whole point of the question. Make use of Google rather than just asking others to do it for you.

Br because it is the most electronegative

NEVER use electronegativity to explain ionization energy. Electronegativity of a certain atom is defined as the attractive force of the atom towards the bonding electrons. However, NO bond is formed in ionization of gaseous atoms.