A coordinate covalent bond is a type of covalent bond in which both electrons in the shared pair come from the same atom. In the ammonium ion (NH\textsubscript{4}\textsuperscript{+}), one lone pair from nitrogen is donated to a hydrogen ion (H\textsuperscript{+}), forming a coordinate bond. The other three N–H bonds are regular covalent bonds.
Other options:
(a) HCl — polar covalent bond.
(b) NH\textsubscript{3} — only covalent bonds.
(c) H\textsubscript{2}O — only covalent bonds.