Answer: [tex]NH_3[/tex]
Explanation: According to Arrhenius concept, a base is defined as a substance which donates hydroxide ions [tex](OH^-)[/tex] when dissolved in water and an acid is defined as a substance which donates hydronium ions [tex](H_3O^+)[/tex] in water.
According to the Bronsted Lowry conjugate acid-base theory, an acid is defined as a substance which donates protons and a base is defined as a substance which accepts protons.
According to the Lewis concept, an acid is defined as a substance that accepts electron pairs and base is defined as a substance which donates electron pairs.
Thus [tex]NH_3[/tex] is a lewis base as it contains a lone pair of electrons and thus can donate it.
[tex]NH_3+H^+\rightarrow NH_4^+[/tex]
