To determine the best estimate for the number of COVID-19 cases in the UK on day 20 using the exponential fit line from the graph, you would need to look at where day 20 falls on the x-axis and see what the corresponding value is on the y-axis at that point.
Based on the typical growth patterns in such graphs and the choices provided, it seems that a figure around 10,000 cases would be a reasonable estimate for exponential growth during that time frame. However, without the actual graph, I can only suggest that you refer to the line at day 20 to make a precise determination.
Therefore, the most logical estimate for the number of COVID-19 cases on day 20 would be 10,000 cases if that aligns with the line on the graph.