How should a mean reaction time be calculated from four valid trials?

The correct option is Add the four results and divide by four. This is the best answer because it directly matches the learning objective to apply safe and valid practical design to reaction-time investigations, including control variables, repeats and calculation of mean values.

The correct option is Add the four results and divide by four. Add the four results and divide by four supports the answer because it fits Structure and function in The human nervous system and uses the required AQA GCSE Biology context. The objective asks students to apply safe and valid practical design to reaction-time investigations, including control variables, repeats and calculation of mean values, so the explanation must connect the option to that exact skill or idea. The other options are incorrect because "Use only the fastest result", "Subtract the smallest result from the largest result", "Multiply the first result by the number of repeats" do not match the specific approved objective or practical context.