Answer:

A Lewis base is a species that donates an electron pair in a chemical reaction. In other words, it is a substance that can accept a proton or donate an electron pair to form a coordinate covalent bond. Among the given options, ammonia (NH3) is a Lewis base.

Explanation:

Definition of Lewis Base:
A Lewis base is a substance that donates an electron pair in a chemical reaction. It is also known as an electron pair donor. Lewis bases are characterized by having at least one lone pair of electrons that can be donated to form a coordinate covalent bond.

Analysis of the Given Options:
a) Ammonia (NH3): Ammonia is a Lewis base because it has a lone pair of electrons on the nitrogen atom that can be donated to form a coordinate covalent bond. It can act as a Lewis base by donating its lone pair of electrons to a Lewis acid.

b) Magnesium chloride (MgCl2): Magnesium chloride is not a Lewis base because it does not have any lone pairs of electrons available for donation. It is a compound formed by the transfer of electrons from magnesium to chlorine, resulting in the formation of ionic bonds.

c) Aluminium chloride (AlCl3): Aluminium chloride is not a Lewis base because it does not have any lone pairs of electrons available for donation. It is an example of a Lewis acid, which is a species that accepts an electron pair.

d) Sodium ion (Na+): Sodium ion is not a Lewis base because it does not have any lone pairs of electrons available for donation. It is a cation formed by losing an electron, and cations are generally not Lewis bases.

Conclusion:
Based on the analysis of the given options, the correct answer is option 'A' - ammonia (NH3). Ammonia is a Lewis base because it has a lone pair of electrons available for donation, which allows it to act as an electron pair donor in chemical reactions.