The redox reaction among the given options is the combination of dinitrogen (N2) with dioxygen (O2) at 2000 K.

Explanation:
Redox reactions involve the transfer of electrons between species. In these reactions, one species is oxidized (loses electrons) while another species is reduced (gains electrons). Let's analyze each option to identify the redox reaction:

a) Reaction of [Co(H2O)6]Cl3 with AgNO3:
In this reaction, the cobalt complex [Co(H2O)6]3+ is displaced by the silver ion, resulting in the formation of [Ag(H2O)6]3+ and Cl- ions. There is no change in the oxidation state of any element, so this is not a redox reaction.

b) Formation of ozone from atmospheric oxygen in the presence of sunlight:
The formation of ozone (O3) from oxygen (O2) in the presence of sunlight is a photochemical reaction. It involves the splitting of O2 molecules into individual oxygen atoms, which then combine with other O2 molecules to form ozone. This reaction does not involve any change in the oxidation state of oxygen, so it is not a redox reaction.

c) Combination of dinitrogen with dioxygen at 2000 K:
This is the redox reaction among the given options. At high temperatures (2000 K), nitrogen gas (N2) can react with oxygen gas (O2) to form nitrogen dioxide (NO2). In this reaction, nitrogen undergoes oxidation from an oxidation state of 0 in N2 to +4 in NO2, while oxygen undergoes reduction from an oxidation state of 0 in O2 to -2 in NO2.

d) Reaction of H2SO4 with NaOH:
When sulfuric acid (H2SO4) reacts with sodium hydroxide (NaOH), a neutralization reaction occurs. The hydrogen ions (H+) from sulfuric acid combine with the hydroxide ions (OH-) from sodium hydroxide to form water (H2O), and the remaining sodium ion (Na+) and sulfate ion (SO42-) combine to form sodium sulfate (Na2SO4). There is no change in the oxidation state of any element, so this is not a redox reaction.

In summary, the redox reaction among the given options is the combination of dinitrogen with dioxygen at 2000 K.