To determine the best estimate for the number of COVID-19 cases in the UK on day 22 using the graph, you would need to look at the corresponding value on the y-axis for day 22 on the x-axis.
Unfortunately, as I cannot see the graph, I can’t provide a direct answer from the options given (3,500, 25,000, 30,000, or 21,000 cases). You would need to refer to the graph and find the value at day 22 to make the best estimate.
If you give me the numerical value shown on the graph for day 22, I can help confirm if one of the options is correct!