Electron affinity is positive, when:(a) O changes to {{O}^ {-}} (b) {{O}^ {-}} changes into {{O}^ {2-}} (c) O changes into {{O}^ {+}} (d) electron affinity is always negative

Electron affinity is defined as the energy released when an atom gains an electron to form a negative ion. It's typically a negative value since energy is released when an atom gains an electron.

Therefore, the correct option is (a) O changes to
�
−
O
−
. When oxygen (O) changes to
�
−
O
−
, it gains an electron, releasing energy, and the electron affinity is positive (though the energy change is negative).

In option (b),
�
−
O
−
changing to
�
2
−
O
2−
involves gaining another electron, releasing more energy, thus reinforcing the negative electron affinity.

Option (c) suggests O changes into
�
+
O
+
, which would require the removal of an electron, resulting in a positive electron affinity but that's not the common behavior for oxygen.

Option (d) is incorrect since electron affinity is not always negative; it can be positive for certain elements under specific conditions, as mentioned in option (a).