A,B  and C  are elements in the third period. Oxide of A is ionic and basic, that of B is amphoteric and of C is a giant molecule. A, B and C will have atomic numbers in the order

Since oxide of the element A is ionic and basic, therefore, the element A can be alkali metal (group 1 such as Na). Oxide of B is amphoteric, the element B can be from p− block such as group 13 (All). The oxide of C is a giant molecule and can have a three dimensional network solid such as SiO2  (C from group 14). Thus, elements A, B and C have atomic numbers in the order: