Answer is (2) - hydrogen carbonate
Step-by-step explanation:
NaHCO₃ is an ionic compound which is made from Na⁺ and HCO₃⁻ ions. The decomposition is
NaHCO₃ → Na⁺ + HCO₃⁻
Among the resulted ions, Na⁺ is a monatomic ion while HCO₃⁻ is a polyatomic ion.
Polyatomic ions mean ions which are made of two or more different atoms.
HCO₃⁻ is made from 3 atoms as H, C and O. The name of HCO₃⁻ ion is bicarbonate or hydrogen carbonate.