I think the first expert gave a wrong answer (he said reaction 6 and NO2 is being reduced from -1 to 0): Reduction is a gain of an electron so NO2 would not be reduced from -1 to 0.

Which one of the 6 reactions involving metal ions represents a redox reaction? Which species is oxidized (write the element and the oxidation number before and after oxidation)? Which species is reduced (write the element and the oxidation number before and after reduction)? These are the following 6 reactions:

1) Ag+ (aq) + HCl (aq) AgCl (s, white) + H+ (aq)

2) AgCl (s) + 2 NH3 (aq) Ag(NH3)2+ (aq) + Cl (aq)

3) Fe3+ (aq) + 3 NH3 (aq) + 3 H2O (l) Fe(OH)3 (s) + 3 NH4+ (aq)

4) Fe3+ (aq) + SCN (aq) Fe(SCN)2+ (aq, red-brown)

5) 4 Fe3+ (aq) + 3 K4[Fe(CN)6] (aq) Fe4[Fe(CN)6]3 (s, blue) + 12 K+

6) Co2+ (aq) + 7 NO2 (aq) + 3 K+ (aq) + 2 H+ (aq) NO (g) + H2O (l) + K3[Co(NO2)6] (s, yellow)