If a good decreases in quantity as more people consume it, it is characterized as:

A good that decreases in quantity as more people consume it is characterized as rival in consumption. This means that one person's use of the good diminishes the ability of another person to use it. For instance, when a pizza is consumed, each slice taken by a person reduces the total amount of pizza available for others.

This concept is fundamental in understanding how resources are allocated and consumed within an economy. When a good is classified as rival, it highlights the competitive nature of consumption where individuals or entities compete for the same resource.

In contrast, the other classifications, such as non-excludable, public good, and private good, do not inherently focus on the decrease in quantity as more people consume them in the same way. Non-excludable goods are those that are difficult to prevent anyone from accessing, while public goods are both non-excludable and non-rivalrous, meaning consumption by one does not reduce availability for others. Private goods, while they can be rivalrous, do not necessarily fit the definition related to the decrease in quantity with increased consumption by others in the same manner. Hence, rival in consumption effectively describes this scenario.