Answer: \(NH_3\)
Explanation: According to Arrhenius concept, a base is defined as a substance which donates hydroxide ions \((OH^-)\) when dissolved in water and an acid is defined as a substance which donates hydronium ions \((H_3O^+)\) 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 \(NH_3\) is a lewis base as it contains a lone pair of electrons and thus can donate it.
\(NH_3+H^+\rightarrow NH_4^+\)
