Final answer:
The decrease in the water's property in the vat is likely referring to a decrease in (a) volume due to thermal contraction, which results from a decrease in water's temperature.
Step-by-step explanation:
The question presents a scenario where a vat of 7.39 kg of water undergoes a decrease in a certain property. Without additional context, it's challenging to determine what specific property is being referred to since water's volume, temperature, pressure, and density can all change under different conditions.
However, touching on the relationship between these properties and phases of water, pressure and temperature can affect the state of water as described in the given information snippets.
A change in temperature of water can cause thermal expansion or contraction leading to volume changes, while variations in pressure can lead to phase changes as hinted at by the snippets about water behavior under differing pressures and temperatures. Thus, if the vat underwent a decrease in temperature, this would generally lead to a decrease in volume, assuming the pressure remains constant.
However, based on the provided context, an assumption can be made that the question more likely refers to the thermal contraction of water which would be resulted by a decrease in temperature. This is aligned with the concept that most materials, including water, tend to decrease in volume as the temperature drops, explained as thermal contraction.