Answer : The correct option is, [tex]N_2+3H_2\rightarrow 2NH_3[/tex]
Explanation :
(1) [tex]CuSO_4+BaCl_2\rightarrow BaSO_4+CuCl_2[/tex]
(2) [tex]Pb(NO_3)_2+2NaCl\rightarrow PbCl_2+2NaNO_3[/tex]
(3) [tex]K_2SO_4+BaCl_2\rightarrow BaSO_4+2KCl[/tex]
The reactions 1, 2, and 3 are the example of double-displacement reaction in which the cation and anion of the two reactants exchange their places to give two new compounds.
(4) [tex]N_2+3H_2\rightarrow 2NH_3[/tex]
This reaction is an example of oxidation-reduction reaction.
Oxidation reaction : It is defined as the reaction in which a substance looses its electrons. In this, oxidation state of an element increases.
Reduction reaction : It is defined as the reaction in which a substance gains electrons. In this, oxidation state of an element decreases.
In this reaction the oxidation state of nitrogen and hydrogen is (0) and the oxidation state of nitrogen and hydrogen in [tex]NH_3[/tex] are, (-3) and (+1). So, from this we conclude that the nitrogen shows reduction reaction and hydrogen shows oxidation reaction.
Hence, the correct option is, [tex]N_2+3H_2\rightarrow 2NH_3[/tex]
