Alk aline earth metal (A), belongs to 3rd period reacts with oxygen and nitrogen to form compound (B) and (C) respectively. It undergo metal displacement reaction with `AgNO_3` solution to form compound (D).

Explanation

(i) Alkali earth metal (A) belonging to `3^(rd)` period is magnesium.
(ii) Magnesium reacts with oxygen and nitrogen to form magnesium oxide (B) and magnesium nitride (C).
`2Mg+O_2tounderset((B))2MgO`
`3Mg+N_2tounderset((C ))Mg_3N_2`
Magnesium undergoes metal displacement reaction with `AgNO_3` to form magnesium nitrate (D).
`Mg+2AgNO_3tounderset((D))Mg(NO_3)_2+2Ag`