The correct group of halide ions which can be oxidised by oxygen in acidic medium is

We need to identify which halide ions can be oxidised by oxygen in acidic medium.

The reduction potential of O$$_2$$ in acidic medium:

$$O_2 + 4H^+ + 4e^- \to 2H_2O$$, $$E° = +1.23$$ V

For oxidation of X$$^-$$ to X$$_2$$:

- $$2Cl^- \to Cl_2 + 2e^-$$, $$E° = -1.36$$ V (very difficult to oxidise)

- $$2Br^- \to Br_2 + 2e^-$$, $$E° = -1.07$$ V

- $$2I^- \to I_2 + 2e^-$$, $$E° = -0.54$$ V (easiest to oxidise)

For a reaction to be spontaneous: $$E°_{cell} = E°_{cathode} - E°_{anode} > 0$$

For I$$^-$$: $$E°_{cell} = 1.23 - 0.54 = 0.69$$ V > 0 $$\checkmark$$ (spontaneous)

For Br$$^-$$: $$E°_{cell} = 1.23 - 1.07 = 0.16$$ V > 0, but this is marginal and in practice O$$_2$$ does not readily oxidise Br$$^-$$ in dilute acidic solutions.

For Cl$$^-$$: $$E°_{cell} = 1.23 - 1.36 = -0.13$$ V < 0 $$\times$$ (not spontaneous)

In practice, only iodide is readily oxidised by O$$_2$$ in acidic medium. This is why KI solutions turn yellow/brown on standing in air.

The correct answer is Option C: I$$^-$$ only.