To determine the best estimate for the number of COVID-19 cases in the UK on day 26 using the graph, you would need to look at the point on the graph that corresponds to day 26. Given that I don’t have access to the actual graph, I can't provide an exact number.
However, you can visually check the value on the y-axis at the point of day 26 and see which option (8,000 cases, 15,000 cases, 3,500 cases, or 20,000 cases) aligns most closely with the y-axis value for that day.
If you're able to analyze the graph now, choose the response that matches the y-value at day 26 for the most accurate estimate.