When moist coloured flowers are added into \(\mathrm{SO}_{2}\) gas, the flowers are decolourised because (a) \(\mathrm{SO}_{2}\) oxidises vegetable colouring matter (b) \(\mathrm{SO}_{2}\) absorbs colouring matter (c) \(\mathrm{SO}_{2}\) gives colourless product (d) \(\mathrm{SO}_{2}\) reduces vegetable colouring matter

Sulfur dioxide (SO2) exhibits distinct chemical behavior as a reducing agent. This means it has a tendency to donate electrons to other substances during chemical reactions. In simple terms, SO2 can chemically 'trap' oxygen from compounds, effectively reducing their oxidation state. This characteristic allows SO2 to facilitate various chemical processes, including the preservation of food, removal of chlorine from industrial waste, and even processing of photographic films.

In terms of chemical reactions, when the term 'reducing agent' is mentioned, it implies a substance like SO2 which loses electrons and becomes oxidized in the process, thus allowing the other substance to gain those electrons and become reduced. This electron exchange can heavily alter the properties of the other reactants, often leading to significant changes like the decolourisation of pigments.

The interaction between SO2 and pigments is a fascinating example of its reducing power. Pigments in flowers and other colored matter are typically composed of complex organic compounds. When exposed to sulfur dioxide, especially in a moist environment, these compounds can undergo a reduction reaction.

This reaction usually entails a change in the molecular structure of the pigmented substances, often resulting in a different arrangement or the removal of double bonds that are responsible for their vibrant colors. As a consequence, the substances either turn into a colorless variety or change in such a manner that their ability to reflect specific wavelengths of light, and hence their visible color, is lost. This process is not an absorption by SO2 but a chemical alteration due to SO2's reducing action on the compounds.

When discussing the decolourisation of flowers by SO2, it is important to understand that this phenomenon is a direct consequence of the reduction of pigments, as mentioned earlier. In the presence of moisture, SO2 becomes more reactive, forming sulfurous acid which then interacts with the pigments found in flowers.

The decolourisation is because the sulfur dioxide reacts with the substances responsible for the flowers' coloration, reducing them and thus bleaching the flowers. This process essentially strips the flowers of their colorful pigments, rendering them colorless. It demonstrates the reducing property of SO2 in action, which is why, in the given multiple-choice question, the correct answer is indeed (d) SO2 reduces vegetable colouring matter - leading to the flowers' decolourisation.