To determine which activity uses the least amount of water in households according to the diagram, you would typically need to refer to the specific statistics or information depicted in that diagram. However, if I had to make a general assumption based on common water usage trends, flushing the toilet often uses less water compared to washing clothes or taking showers. The dishwasher can vary, but it may use more or similar amounts as a shower.
Based on common knowledge, I would suggest that the answer is likely C: flush the toilet. However, please review the specific diagram for an accurate answer.