Cereal grains (wheat, rice, maize, barley) consist of $70 - 80\%$ carbohydrates by dry weight, predominantly composed of complex polysaccharides. Starch (a high-molecular-weight glucose polymer of amylose and amylopectin) constitutes $60 - 75\%$ of the grain, complemented by non-starch structural cell-wall polysaccharides (dietary fiber such as cellulose, arabinoxylans, and $eta$-glucans). Free mono- and disaccharides account for less than $2 - 3\%$.

Option analysis:

• Monosaccharides: Present only in trace amounts in mature grain.
• Disaccharides: Minor components (sucrose $\lt 1 - 2\%$).
• Polysaccharides: Correct, starch and fiber polysaccharides form the overwhelming majority of cereal carbohydrates.
• Oligosaccharides: Present in minor quantities (raffinose, fructo-oligosaccharides).

Transition metal pro-oxidants catalyze lipid peroxidation and flavor reversion by decomposing hydroperoxides into reactive free radicals. Transition metals with readily alternating redox states, particularly copper ($Cu^{2+}/Cu^+$) and cobalt ($Co^{3+}/Co^{2+}$) or iron, are extremely potent oxidation catalysts even in parts-per-billion concentrations. In contrast, nickel ($Ni$) is significantly less active as an oxidation/reversion accelerator, which is why finely divided nickel is safely employed as the heterogeneous catalyst in commercial vegetable oil hydrogenation without catalyzing immediate catastrophic reversion.

Option analysis:

• Copper: Most potent pro-oxidant metal catalyst in oils.
• Cobalt: Very active pro-oxidant catalyst.
• Zinc: Pro-oxidant activity lower than Cu/Co but higher than Ni in many lipid media.
• Nickel: Correct, least active accelerator of oil flavor reversion among the choices.