Final answer:
The correct unit for total crop yield is t/ha or t , ha^-1 as it represents the mass of harvested crops per unit area.
Step-by-step explanation:
The correct unit for total crop yield in the calculation is t/ha or t , ha-1.
This is because it represents the mass of harvested crops per unit area and accounts for the field yield and area. The unit t/ha indicates that for every hectare of land, a certain mass of crops (in metric tons) is produced.
Therefore, the correct answer is a. It represents the mass of harvested crops per unit area.