Final answer:
The stable allotrope of sulfur at room temperature is rhombic sulfur. It is the most common and stable form and other allotropes revert to it at room temperature.
Step-by-step explanation:
The question is asking to identify the stable allotrope of sulfur based on the enthalpy of combustion for rhombic and monoclinic sulfur. The enthalpy of combustion for rhombic sulfur is -297.5 kJ/mol, and for monoclinic sulfur, it is -300 kJ/mol.
According to the given information, rhombic sulfur (Sa) is stable below 95.3°C, while monoclinic sulfur (Sß) is stable above this temperature. Since the formation of monoclinic sulfur is an endothermic process, it absorbs energy to convert from rhombic to monoclinic. Yellow, rhombic sulfur is the most common and most stable form at room temperature. It reverts other allotropes to its form at room temperature, making it the stable allotrope.
The fact that the enthalpy of combustion is slightly higher (more negative) for monoclinic sulfur suggests it is slightly less stable compared to rhombic sulfur, as it releases slightly more energy upon combustion. Thus, the stable allotrope of sulfur at room temperature is rhombic sulfur.