Which one of the following cannot function as an oxidising agent?

To determine which species cannot function as an oxidizing agent, we must recall that an oxidizing agent accepts electrons and gets reduced in a redox reaction. Therefore, a species that cannot be reduced (i.e., cannot gain electrons) cannot act as an oxidizing agent. We will examine each option by checking if it can be reduced.

Starting with Option A: I$$^-$$. The iodine atom in I$$^-$$ has an oxidation state of -1, which is the lowest possible oxidation state for iodine. Since it cannot accept more electrons to be reduced further, I$$^-$$ cannot act as an oxidizing agent. Instead, it tends to lose electrons and act as a reducing agent.

Next, Option B: S(s). Sulfur in its elemental form has an oxidation state of 0. Sulfur can be reduced to lower oxidation states, such as -2 in compounds like H$$_2$$S. For example, in the reaction with iron: $$Fe + S -> FeS$$, sulfur is reduced from 0 to -2. Thus, S(s) can function as an oxidizing agent.

Option C: NO$$_3^-$$ (aq). The nitrogen atom in nitrate ion has an oxidation state of +5. Nitrogen can be reduced to lower oxidation states, such as +4 in NO$$_2$$, +2 in NO, 0 in N$$_2$$, or -3 in NH$$_3$$. For instance, in acidic conditions, nitrate oxidizes copper: $$Cu + 4H+ + 2NO_{3}- -> Cu^{2+ + 2NO2 + 2H2O}$$, where NO$$_3^-$$ is reduced. Hence, NO$$_3^-$$ can act as an oxidizing agent.

Option D: Cr$$_2$$O$$_7^{2-}$$. The chromium atoms in dichromate ion have an oxidation state of +6. Chromium can be reduced to lower oxidation states, such as +3 in Cr$$^{3+}$$. For example, in acidic medium, dichromate oxidizes Fe$$^{2+}$$: $$Cr_{2}O_{7}^{2- + 6Fe^{2+} + 14H+ -> 2Cr^{3+} + 6Fe^{3+} + 7H2O}$$, where Cr$$_2$$O$$_7^{2-}$$ is reduced. Therefore, Cr$$_2$$O$$_7^{2-}$$ can function as an oxidizing agent.

From the analysis, I$$^-$$ (Option A) cannot be reduced and thus cannot act as an oxidizing agent, while the other options can. Hence, the correct answer is Option A.