Which of the following cannot function as an oxidising agent?

We need to identify which species cannot function as an oxidising agent.

Key Concept: An oxidising agent accepts electrons (gets reduced). For this, the element must have the ability to decrease its oxidation state.

$$N^{3-}$$: Nitrogen is in its lowest possible oxidation state (-3). Since it cannot be reduced further (there is no lower oxidation state for nitrogen), it cannot act as an oxidising agent. It can only act as a reducing agent (losing electrons to go to higher oxidation states).

$$SO_4^{2-}$$: Sulphur is in +6 state. It can be reduced (e.g., to $$SO_2$$, $$S$$, or $$H_2S$$). It can act as an oxidising agent, especially in concentrated $$H_2SO_4$$.

$$BrO_3^-$$: Bromine is in +5 state. It can be reduced to $$Br^-$$ (0 or -1). It is a known oxidising agent.

$$MnO_4^-$$: Manganese is in +7 state. It is one of the strongest oxidising agents, capable of being reduced to $$Mn^{2+}$$, $$MnO_2$$, or $$MnO_4^{2-}$$.

The correct answer is Option 1: $$N^{3-}$$.