Final answer:
A gas confined to a rigid container that cannot expand as heat is added to it undergoes an isochoric, or isometric, process (D), where the volume of the system remains constant.
Step-by-step explanation:
When a gas is confined to a rigid container that cannot expand as heat is added to it, this process is referred to as an isochoric process. In an isochoric process, the volume of the system does not change. According to the provided information and the description of various thermodynamic processes, the answer here is D) isometric, which is synonymous with isochoric in the context of thermodynamics. Unlike adiabatic processes, which involve no heat transfer (Q=0), or isothermal processes, where the temperature remains constant, an isochoric process is characterized by a constant volume. It is worth noting that isometric and isochoric are often used interchangeably in thermodynamics to describe this process.