**The Species and their Electron Configurations**

To determine which species is the easiest to remove one electron from, we need to consider their electron configurations. The species in question are:

a. O(g) - Oxygen atom
b. O2- - Oxygen anion
c. O- - Oxygen monoanion
d. O - Oxygen cation

**Electron Configurations**

The electron configurations of these species are as follows:

a. O(g) - 1s^2 2s^2 2p^4
b. O2- - 1s^2 2s^2 2p^6
c. O- - 1s^2 2s^2 2p^5
d. O - 1s^2 2s^2 2p^4

**Explanation**

When removing an electron from an atom or ion, we are essentially breaking a bond between the electron and the nucleus. The energy required to remove an electron is known as the ionization energy. The lower the ionization energy, the easier it is to remove an electron.

In the given species, the electron configuration of O2- (oxygen anion) is 1s^2 2s^2 2p^6. This means that the oxygen anion already has a stable, filled electron configuration with 8 valence electrons. Removing one electron from the 2p orbital would result in a stable electron configuration with 8 valence electrons, similar to a noble gas configuration.

The other species, O(g) (oxygen atom), O- (oxygen monoanion), and O (oxygen cation), all have incomplete electron configurations in the 2p orbital. Removing one electron from these species would result in an unstable electron configuration, causing an increase in the ionization energy required.

Therefore, O2- is the easiest to remove an electron from because it already has a stable electron configuration with 8 valence electrons. The additional electron is loosely held and requires the least amount of energy to remove.

Note: The explanation provided here is based on the principles of electron configurations and ionization energies. These concepts are fundamental in chemistry and can be further explored for a more comprehensive understanding.