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

Strategy: Use standard reduction potentials to determine which halide ions can be oxidized by Oxygen.

Step 1: Redox Potentials In acidic medium, the reduction half-reaction for Oxygen is: $\ce{O2 + 4H^+ + 4e^- \to 2H2O} \quad (E^\circ = +1.23 \text{ V})$ For halides $(\ce{X2/X^-})$:

• $\ce{Cl2/Cl^-}: +1.36 \text{ V}$
• $\ce{Br2/Br^-}: +1.09 \text{ V}$
• $\ce{I2/I^-}: +0.54 \text{ V}$

Step 2: Feasibility Oxygen can spontaneously oxidize species with $E^\circ < +1.23 \text{ V}$. This includes $\ce{I^-}$ (readily) and $\ce{Br^-}$. However, the reaction with $\ce{I^-}$ is fast and thermodynamically very favorable: $\ce{4I^- + 4H^+ + O2 \to 2I2 + 2H2O}$ In many practical contexts (and standard textbook problems), Oxygen is specifically cited as oxidizing $\ce{I^-}$ primarily in acidic medium.

$\boxed{\text{Answer: (Option) [c] $\ce{I^-}$ only}}$