Which of the following has the highest electron affinity?(A) K (B) {{O}^{-}} (C) {{F}^{-}} (D) O

Among the given options, {{F}^{-}} (fluoride ion) has the highest electron affinity.

Electron affinity is the energy change that occurs when an atom or ion in the gaseous state gains an electron to form a negatively charged ion (anion). It is a measure of the attraction between the incoming electron and the nucleus of the atom or ion.

Fluorine (F) is located in the halogen group of the periodic table, and halogens generally have very high electron affinities because they only need one more electron to achieve a stable noble gas electron configuration. As we move from left to right across a period in the periodic table, the electron affinity generally increases due to the increasing nuclear charge, which results in a stronger attraction for an additional electron.

Thus, out of the given options, {{F}^{-}} (fluoride ion) has the highest electron affinity.