Given that A turns blue litmus red. So A is acidic in nature.
Brisk effervescence with NaHCO3 indicates carbon dioxide as one of the products.
The molecular formula of A contains 2 oxygen atoms. So the compound is likely to be a carboxylic acid (general molecular formula is CnH2nO2)
Here n=2 and hence the compound A is ethanoic acid. The reaction is given by:
CH3COOH + NaHCO3→ CH3COONa + H2O + CO2