Reduction is defined in terms of electronation and hydrogenation

Reduction is a chemical process that involves the gain of electrons or a decrease in oxidation number. It can be defined in terms of two main reactions: electronation and hydrogenation.

I. Electronation:
- Electronation refers to the process of gaining electrons. In a reduction reaction, a molecule, ion, or atom accepts one or more electrons, resulting in a more negative charge.
- For example, in the reaction: X + e- -> X-, the atom X gains an electron, and its oxidation number decreases. This is a reduction reaction.

II. Hydrogenation:
- Hydrogenation is a specific type of reduction reaction where hydrogen atoms are added to a molecule.
- During hydrogenation, the molecule gains hydrogen atoms, and its oxidation number decreases. This is a reduction reaction.
- For example, in the reaction: X + H2 -> XH2, the molecule X gains hydrogen atoms, and its oxidation number decreases. This is a reduction reaction.

III. Increase in oxidation number:
- Increase in oxidation number is not associated with reduction but with oxidation.
- In oxidation, the oxidation number of an atom, ion, or molecule increases, indicating the loss of electrons.
- Reduction, on the other hand, involves the gain of electrons, resulting in a decrease in oxidation number.

IV. Decrease in oxidation number:
- Decrease in oxidation number is directly associated with reduction.
- Reduction reactions involve the gain of electrons, leading to a decrease in oxidation number.
- For example, if the oxidation number of an atom changes from +2 to 0, it has undergone reduction because its oxidation number has decreased.

Conclusion:
Based on the definitions of reduction and the given options, the correct terms that define reduction are I. electronation and IV. decrease in oxidation number. These two terms accurately describe the process of reduction, where electrons are gained and the oxidation number decreases. Therefore, option A. (I and IV) is the correct answer.