Final answer:
The process described, where a gas expands at constant pressure as heat is added, is known as an isobaric process.
Step-by-step explanation:
When a gas is allowed to expand at constant pressure as heat is added to it, the process is called an isobaric process. This is because the term "isobaric" refers to a process that occurs at a constant pressure. In contrast, an adiabatic process would be one in which no heat is transferred to or from the gas, an isometric (or isochoric) process would occur at constant volume, an isothermal process would be at constant temperature, and an isentropic process would be both adiabatic and reversible. Therefore, for the scenario described, the correct option is A) isobaric.