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) An alkaline earth (A) metal belongs to third period is magnesium (Mg)
(ii) Magnesium reacts with oxygen to form magnesium oxide (MgO) (B)
`2Mg +O_(2)tounderset(Magnesium oxide)(2MgO_(2))`
(iii) Magnesium reacts with nitrogen to form magnesium nitride `Mg_(3)N_(2)(C)`
`3Mg+N_(2)O_(2)tounderset("Magnesium nitride")(Mg_(3)O)`
(iv) Magnesium undergoes metal displacement reaction with `AgNO_(3)` solution to form magnesium nitrate `Mg(NO_(3))_(2) (D)`
`Mg+2AgNO_(3) tounderset("Magnesium nitrate") (Mg(NO_(3))_(2) +)2Ag`