BeO does not dissolve in H2O but being an amphoteric dissolve in NaOH. On moving down in a group, solubility of alkaline earth metal in water decreases. Oxides and hydroxides of alkaline earth metal are basic except Be which is amphoteric. Hence, the metal M is Be. It forms beryllium sulphate (BeSO4) which is water soluble. It forms beryllium hydroxide [Be(OH)2] which is water insoluble. Be(OH)2 being amphoteric reacts with NaOH.