Final answer:
The correct unit for calculating the surface area of the barn is either square meters (m²) if using the Metric system, or square feet (ft²) if using the Imperial system. So the correct answer is option B.
Step-by-step explanation:
To calculate the surface area of the barn, the farmer would use the unit that measures area. The correct unit for surface area in the Metric system is square meters (m²), and in the Imperial system, it is square feet (ft²). Thus, the correct choice from the given options would be B. m² if using Metric, or D. ft² if using Imperial measurements. Area is the measure of the extent of a two-dimensional surface enclosed within a boundary, and it is represented by square units, reflecting the concept of length times width.