Which design best tests the effect of light direction on seedling growth?

The correct option is Expose seedlings to light from one side while keeping water, temperature, and seedling age the same. This is the best answer because it directly matches the learning objective to apply safe and valid practical design to plant-response investigations, including control variables, repeats, light direction and gravity treatment.

The correct option is Expose seedlings to light from one side while keeping water, temperature, and seedling age the same. Expose seedlings to light from one side while keeping water, temperature, and seedling age the same supports the answer because it fits Control and coordination in Plant hormones (biology only) and uses the required AQA GCSE Biology context. The objective asks students to apply safe and valid practical design to plant-response investigations, including control variables, repeats, light direction and gravity treatment, so the explanation must connect the option to that exact skill or idea. The other options are incorrect because "Use different plant species in every test group", "Change the light direction, water volume, and temperature all at once", "Use one seedling only and record no measurements" do not match the specific approved objective or practical context.