The correct answer is (a) -3, 5
To explain: Dividing into two parts oxidation number of N in NH4^+ is x; x + 4*1 = +1; x = -3, oxidation number of N in NO3^– is y; y + 3*-2 = -1; y = 5. The oxidation States of Nitrogen in this compound are -3 and 5 respectively.