An artesian aquifer could occur in a(n):

An artesian aquifer is characterized by water that is under pressure between layers of impermeable material. This pressure can cause the water to rise to the surface without the need for pumping, often resulting in a flowing well.

When it comes to the different types of aquifers listed, a confined aquifer is the only one that meets the criteria for an artesian system. In a confined aquifer, the water is trapped between two layers of impermeable rock or sediment, making it possible for the water to become pressurized.

In contrast, unconfined aquifers allow the water table to rise and fall freely with the surface conditions, meaning that the water is not under pressure in the same way. While a water table aquifer is also another term that refers to unconfined aquifer systems, they are similarly not conducive to the artesian condition. Additionally, a shale formation does not inherently mean it can serve as an artesian aquifer; while shale can be impermeable and can trap water, it is not typically considered a source of artesian water on its own.

Therefore, the defining characteristics of a confined aquifer make it the correct answer for the conditions under which an artesian aquifer can occur.