To determine the best estimate for the number of COVID-19 cases in the UK on day 20 using the graph, we would need to visually inspect the graph or analyze the exponential fit line for day 20 (which corresponds to March 15).
However, since I'm unable to see the graph directly, the typical method to reach an answer would involve finding where day 20 falls on the x-axis and then identifying the corresponding value on the y-axis.
If you can observe the graph, please find the y-coordinate that corresponds to day 20. If I had to make an estimate without seeing the graph, it would be useful to choose a value that seems consistent with exponential growth commonly seen in COVID-19 cases during that initial stage of the pandemic.
If you have estimates from the graph or can provide additional details, I'd be happy to refine my advice or help you narrow down the options. Based on the options provided, commonly exponential growth in the early days could suggest an estimate around 10,000 cases or 12,000 cases. I suggest checking the graph carefully for the exact correspondence.