Iodide ion is oxidised by chlorine is the correct statement among the given options. Let's understand why this is true.

Oxidation and reduction are two complementary chemical reactions that occur simultaneously. In an oxidation reaction, a species loses electrons, while in a reduction reaction, a species gains electrons. Oxidation and reduction always occur together, forming what is known as a redox reaction.

Let's examine each statement given in the options:

a) Chloride ion is oxidised by O2: This statement is incorrect. Chloride ion (Cl-) is a strong reducing agent and does not get oxidized by molecular oxygen (O2). Instead, it can be oxidized to chlorine gas (Cl2) by stronger oxidizing agents such as manganese dioxide (MnO2) or potassium permanganate (KMnO4).

b) Fe2+ is oxidised by iodide: This statement is incorrect. Iodide ion (I-) is a weaker reducing agent compared to Fe2+. Fe2+ ions can be oxidized to Fe3+ ions by stronger oxidizing agents such as potassium dichromate (K2Cr2O7) or hydrogen peroxide (H2O2), but not by iodide ions.

c) Iodide ion is oxidised by chlorine: This statement is correct. Iodide ion (I-) can be oxidized by chlorine (Cl2) to form iodine (I2) and chloride ions (Cl-). This reaction can be represented by the following equation:

2I- + Cl2 → I2 + 2Cl-

In this reaction, iodide ion is losing electrons and getting oxidized to iodine, while chlorine is gaining electrons and getting reduced to chloride ions.

d) Mn2+ is oxidised by chlorine: This statement is incorrect. Mn2+ ions can act as a reducing agent, but they are not easily oxidized by chlorine. Chlorine is a stronger oxidizing agent and can oxidize Mn2+ ions to Mn4+ ions in the presence of suitable conditions and other reactants, but this reaction is not as common as the oxidation of iodide ions by chlorine.

In conclusion, among the given options, the correct statement is that iodide ion is oxidized by chlorine.