Why are control variables needed in a plant response investigation?

The correct option is They make sure the effect of light direction or gravity is tested fairly. 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 They make sure the effect of light direction or gravity is tested fairly. They make sure the effect of light direction or gravity is tested fairly 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 "They guarantee every seedling grows at exactly the same rate", "They remove the need to repeat the investigation", "They prove that plants do not respond to stimuli" do not match the specific approved objective or practical context.