An acid salt is a salt that contains an acidic ion or proton that can undergo partial hydrolysis in an aqueous solution to form an acidic solution. Among the options given, NaHCO3 is an acid salt because it contains the acidic hydrogen carbonate ion (HCO3-) which can react with water to produce a slightly acidic solution. The other options, NH4CI, MgSO4 7H2O, and CH3COONa, do not contain any acidic ion and therefore are not acid salts.