Final answer:
The student's question regarding the cross-sectional area of a washer cannot be answered conclusively without additional context or details about the washer's dimensions.
Step-by-step explanation:
The question provided is focused on determining the cross-sectional area of a washer. The information given includes several mathematical expressions and physical examples where cross-sectional area plays a crucial role, such as in resistance calculation for a cylinder or changes in area due to temperature variations in blocks. Without the specific dimensions of the washer or the context for the choices (1) a = 16, (2) a = 32, (3) a = 64, (4) a = 128, it's not possible to conclusively determine the cross-sectional area of the washer. Thus, further clarification on the details of the washer is required for a correct calculation.