Answer: Displacement reaction
Step-by-step explanation:
In displacement reactions, an element replaces another element in a compound.
Ca(s)+ 2HNO2 (aq) + Ca(NO2)2 (aq) + H2 (g).
In the reaction above, calcium (Ca) displaces hydrogen (H) from its compound (nitrous acid, HNO2) due to its higher position in the reactivity series than hydrogen. Hence, displaced hydrogen is liberated as gas at the end of the displacement reaction while calcium forms its trioxonitrate salt.
Thus, it is a displacement reaction.