In a redox reaction, oxidation is defined by the:

1.gain of electrons, resulting in an increased oxidation number.
2.loss of electrons, resulting in a decreased oxidation number.
3.gain of electrons, resulting in a decreased oxidation number.
4.loss of electrons, resulting in an increased oxidation number.

Option 4. loss of electrons, resulting in an increased oxidation number.

Explanation:

Oxidation is a process involving loss of electron(s). When this happens the oxidation number of the atom being oxidised increases. This can be seen when calcium (Ca) reacts with chlorine (Cl2) to form calcium chloride (CaCl2) according to the equation given below:

Ca + Cl2 —> CaCl2

The oxidation number of calcium increases from 0 to +2. This implies that calcium is being oxidised as it loses its electrons. The oxidation number of chlorine decreases from 0 to - 1 as it gains electron.

Now, we can see that the oxidation of calcium i.e lose of electrons increased its oxidation number from 0 to +2.

From the simple illustrations above, we can see clearly that oxidation involves loss of electrons, resulting in an increased oxidation number.