Discussion
It is given that the water activity of a food is equal to the relative humidity of the atmosphere that is in equilibrium with the food. The partial pressure of water vapour of the food at a specific temperature is
Answer: A
The partial pressure is equal to the partial pressure of water vapour in air.
Water activity is defined as \(a_{w} = p/p_{0}\), the vapour pressure of water over the food divided by that of pure water at the same temperature. The relative humidity of air is \(p_{v}/p_{0}\) at that temperature. If the food is in equilibrium with the air, \(a_{w}\) equals the equilibrium relative humidity, and since both ratios share \(p_{0}\), the vapour pressures themselves are equal.
Equilibrium also means no net moisture transfer: if the food's vapour pressure were lower it would take up moisture from the air, and if higher it would dry. So 'lesser' and 'greater' describe a food that is not in equilibrium, and 'not dependent' contradicts the definition itself.
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