Ionic bonds comes from the electrostatic attraction between ions of opposite charges.
Bromine is an element that is on the far right of the periodic table in the group of the halogens. It is missing only one electron in its valence shell, so it can easily attract another electron to from a negative ion, Br⁻. Because of the nature of the ionic bond, it usually happens between a metal and a non metal, because non metals of the far right side can normally form stable negative ions and metals can usually form stable positive ions.
From the options, Helium is a noble gas, so it hardly will form ions. Nitrogen and Phosphorous are non metals, that usually form negative ions. Cesium is on the first group of the periodic table, so it is a metal and can easily lose one electron for form a positive cation.
So, it will most likely form an ionic bond with Cesium.