The electronegativity of an atom in a molecule is related to its ionization energy and its electron affinity, which are properties of isolated atoms. The ionization energy measures how tighly the atom binds its electrons, while the electron affinity is a measure of how strongly the atom attracts additional electrons. An atom with a high ionization energy and a strongly negative electron affinity will both attract electrons from other atoms and resist having its electrons taken away; it will be a highly electronegative atom. Since ionization energy increases moving across a period, and electron affinities become more negative moving across a group, we expect electronegativity to increase across a group from left to right.
Since electronegativity follows the periodic trends in ionization energy and electron affinity, we note that when we move down an group, ionization energies decrease and electron affinities become less negative. Thus, we would expect electronegativity to decrease as we move down a group.
this was addressed in part (a).