please explain all the four options
Q.59. Which of the following statements is correct?
(1) Nitrification is a reduction process
(2) The conversion of nitrate into $N{H_4}^{+}$ is denitrification
(3) The conversion of nitrate into ammonia occurs in leaves
(4) Nitrogen fixation is oxidation of atmospheric nitrogen

Dear student,
Answer 3) is correct. The conversion of nitrate into ammonia occurs in leaves.
Because enzyme required for the conversion of nitrate to ammonia occurs in leaves. Example:- nitrite reductase is found in the chloroplast of leaves. 
Explanation of all the three option:-
1) Nitrification is the process of oxidation of ammonia to nitrite and then nitrite to nitrate.
2) Denitrification is the process of formation of reduction of nitrate to nitrogen.
4) In nitrogen fixation atmospheric nitrogen is reduced to ammonia.

Regards